CN1888816A - Extender high-light folding feedback displacement measuring system - Google Patents

Abstract

The beam-expanding strong light folding feedback displacement measurement system belongs to the field of laser measurement technology, and is characterized in that it uses a Zeeman birefringence dual-frequency laser, a beam collimator expander, a hollow corner cube prism, an adjustable attenuator, and a strong reflectivity The feedback mirror constitutes a beam-expanding strong light folding feedback system. When the measured object moves a quarter of the light wavelength along the laser axis, the laser intensity fluctuates periodically, and there are four polarization states in one cycle: o light area, o light and e light coexistence area, e light area and no light area, each area corresponds to one-sixteenth light wavelength displacement of the object, and the displacement of the measured object can be obtained by detecting the number of polarization state areas; when the moving direction of the measured object changes, each period The order in which the four polarization state regions appear in the sensor is also changed, so that the displacement direction of the object can be judged. The device has the characteristics of high precision, large measuring range, easy direction judgment and strong anti-interference ability.

Description

Beam Expanded Strong Light Folding Feedback Displacement Measurement System

technical field

The invention belongs to the technical field of laser measurement.

Background technique

With the rapid development of modern information technology, sensing technology, one of the three pillars of modern information technology, is the foundation of information technology. Its performance determines the function and quality of information system, and its role is becoming more and more important. Displacement is a basic physical quantity in nature, and displacement measurement is widely used in chip manufacturing, automobile industry, biomedicine, aerospace and other fields. In recent years, laser feedback sensing technology and its application have aroused widespread interest of experts and scholars at home and abroad. 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.

At present, the research and application of laser feedback technology are mainly focused on semiconductor lasers. Although a large number of experimental and theoretical results have been obtained by various scientific and technological powers in the world, there are still three very important problems that have not been well resolved, which seriously limit the laser feedback technology. Industrial application of technology development. First, the displacement direction of the measured object cannot be identified. The study found that when the semiconductor laser works at the weak feedback level and the medium feedback level, the feedback signal is an asymmetrical sawtooth wave shape, and the inclination direction of the sawtooth wave is related to the displacement direction of the external reflector. Some researchers try to use this phenomenon to solve the direction judgment problem of semiconductor laser feedback measurement displacement. The inclined direction of the sawtooth wave is easy to judge the displacement direction in terms of perception, but it is not easy to realize in hardware, and at a medium feedback level, the feedback signal will appear hysteresis, which will bring errors to the counting and seriously affect the accuracy of the system measurement. and linearity. When using this method, the feedback signal must be at least one complete cycle before the inclination direction of the feedback waveform can be identified, which limits the resolution of the system measurement to half the optical wavelength, making it impossible to achieve high-precision displacement measurement. At the same time, this type of system has high requirements on the collimation of the feedback cavity and poor ability to resist external interference. Second, the feedback signal cannot be subdivided. Some literatures report that when semiconductor lasers work at extremely weak feedback levels, their feedback signals are cosine-like in shape. Some researchers try to use cosine properties to achieve subdivision similar to the phase measurement technology of traditional laser interferometers. However, this feedback signal is very unstable and easily becomes a sawtooth-like shape. At the same time, the feedback measurement system can only obtain one optical signal, which makes electronic subdivision difficult to achieve. The resolution of the system is generally half an optical wavelength, and the system performance is greatly limited. Third, the feedback external cavity has high requirements for collimation and poor ability to resist external interference. The non-collimation of the feedback external cavity will bring about two influences, one is the influence of multiple feedbacks, and the other is the influence of the system feedback level. Due to the detuning of the external cavity feedback mirror, not only the light reflected once by the feedback mirror can return to the laser resonator to interact with the light in the cavity, but the light reflected twice or even multiple times by the feedback mirror may also return to the laser resonator, modulating the laser. Output, at this time the feedback signal of the system is more complicated, and the measurement system cannot work normally. At the same time, due to the detuning of the external cavity feedback mirror, part of the light cannot return to the laser resonator, which makes the system feedback level drop. Under different feedback levels, the system feedback signals are different, the measurement system cannot obtain the expected feedback signal, and measurement errors occur.

Contents of the invention

The present invention proposes a beam-expanding strong light folding feedback displacement measurement system with high resolution and large range, which can identify the displacement direction of the measured object and has strong anti-detuning and external interference capabilities, which can effectively solve the above-mentioned problems. Three questions.

The present invention is characterized in that it contains:

Beam-expanded strong light folding feedback device, including:

Zeeman birefringence dual-frequency laser, the output frequency difference is less than 40MHz two orthogonal linear polarization frequencies, the laser contains:

Laser gain tube, filled with helium and neon mixed gas;

Quartz crystal wedge, fixed on one side of the above-mentioned laser gain tube; both sides of the quartz crystal wedge are coated with anti-reflection film;

The laser resonator includes a main beam output mirror and a tail beam output mirror; the main beam output mirror is fixed on the other side of the above-mentioned laser gain tube, the inner surface of the main beam output mirror is coated with a strong reflective film, and the upper part of the outer surface is also coated with There is a strong reflection film, and the lower part of the outer surface is coated with an anti-reflection film; the tail beam output mirror is located outside the above-mentioned quartz crystal wedge, and the inner surface of the output mirror is coated with a strong reflection film;

The transverse magnetic field generator is composed of two upper and lower permanent magnets parallel to the central axis of the laser gain tube, the entire gain tube is in the magnetic field, and the direction of the magnetic field is parallel or perpendicular to the polarization direction of any linearly polarized light generated by the above laser ;

The laser feedback external cavity is composed of the inner surface of the above-mentioned main beam output mirror and the upper part of the outer surface of the above-mentioned main beam output mirror, and the main beam output mirror is used as both a laser internal cavity mirror and an external cavity feedback mirror;

The feedback external cavity folder is a hollow corner cube prism, which is composed of three mutually perpendicular glass plates coated with a strong reflection film, placed in the feedback optical path of the above-mentioned laser feedback external cavity, and the feedback light output from the main output end of the laser After passing through the three glass plates, the laser is reflected back to the external cavity by the upper part of the outer surface of the main beam output mirror;

The beam collimation expander is composed of a group of parallel lenses perpendicular to the feedback light output from the main output end of the laser. Beam expansion;

The laser feedback level controller is an absorbing adjustable attenuator, which is placed on one side of the beam collimation expander to make the system work at a strong feedback level;

Signal detection and processing part, including:

The polarizing beam splitter is located outside the above-mentioned tail beam output mirror, and is used to spatially separate the light of two frequencies with orthogonal polarization states output by the tail beam output mirror;

There are two photodetectors, both of which are located outside the above-mentioned polarization beam splitter, and are used to respectively detect the light intensity of the two-frequency light output by the tail beam output mirror;

An amplification and filtering circuit, the input terminals of which are respectively connected to the output terminals of the above two photodetectors, amplify and filter the signals detected by the above photodetectors;

The logic direction determination and counting circuit is sequentially composed of two voltage comparators, two monostable triggers, two forward and reverse NOR gates and a reversible counter in series; the input terminals of the two voltage comparators are respectively connected to the above The output terminals of the amplification and filter circuits are connected, and the input terminals of the two monostable triggers are respectively connected with the output terminals of the above two voltage comparators. Every time the voltage comparator outputs a voltage jump, the monostable trigger generates a pulse signal. Input to two NOR gate elements; when the system moves forward, the positive NOR gate outputs a positive pulse, and the reverse NOR gate has no output; when the system moves in the reverse direction, the reverse NOR gate outputs a reverse Pulse, forward or non-gate has no output; forward and reverse pulse signals are sent to the addition and subtraction terminals of the reversible counter according to the positive and negative, and the magnitude and direction of the object displacement are obtained;

The display circuit, whose input terminal is connected with the output terminal of the above-mentioned logic direction determination and counting circuit, digitally displays the displacement size and direction of the measured object along the laser axis direction;

The base part contains a fixed bracket, which is fixedly connected with the above-mentioned laser gain tube, transverse magnetic field generator, beam collimation expander, laser feedback level controller, laser tail beam output mirror and polarization beam splitter;

The present invention is also characterized in that the above-mentioned quartz crystal wedge is replaced by a glass window plate and a quartz crystal that are coated with an anti-reflection film on both sides, and the glass window plate is fixed on the end of the laser gain tube near the tail beam output mirror. The quartz crystal is fixed on the base and placed between the glass window and the laser tail beam output mirror; A stress generator on the plate is replaced, and the glass window is fixed at the end of the laser gain tube close to the tail beam output mirror.

The displacement measurement system provided by the present invention based on the intense light folding feedback effect of beam expansion in the Zeeman birefringence dual-frequency laser has a resolution of up to one-sixteenth of the light wavelength. For a 632.8nm He-Ne laser, the system resolution It is 39.55nm, and the experimental results show that the measurement range of the device can reach more than 20mm. The displacement measurement system can easily realize the recognition of the displacement direction of the measured object, and has the characteristics of high resolution, large measurement range, strong anti-interference ability and high cost performance.

Description of drawings

Figure 1: One of the implementation examples of the strong light feedback displacement measurement system described in the present invention.

Fig. 2: Schematic diagram of the structure of the main beam output mirror according to the present invention.

Fig. 3: Schematic diagram of the hollow corner cube prism according to the present invention.

Fig. 4: Schematic diagram of the structure of the quartz crystal wedge according to the present invention.

Fig. 5: a block diagram of the logical direction determination and counting circuit of the present invention.

Fig. 6: The second implementation example of the strong light feedback displacement measurement system according to the present invention.

Fig. 7: The third implementation example of the strong light feedback displacement measurement system according to the present invention.

Figure 8: The feedback light intensity curve of two orthogonally polarized lights in the feedback process obtained from the experiment.

Figure 9: The feedback light intensity curve of two orthogonally polarized lights in the feedback process obtained by numerical simulation.

Figure 10: Schematic diagram of the system identifying the displacement direction of the measured object.

Figure 11: The feedback light intensity curve of the system when the measured object produces different displacements. (a) The displacement is 10mm; (b) The displacement is 20mm.

Detailed ways

The invention provides a displacement measuring system with high resolution, large measuring range, capable of identifying the moving direction of the measured object, strong anti-interference ability and high cost performance. The device uses a main beam output mirror whose inner surface is coated with a high-reflection film, the upper part of the outer surface is coated with a high-reflection film, and the lower part is coated with an anti-reflection film, and a hollow corner cube prism constitutes a laser feedback folding system. When the measured object moves along the laser axis by a quarter of the optical wavelength displacement, the light intensity of the laser changes by one fringe, and the resolution of the system is doubled. The feedback level of the system and the collimation of the feedback cavity have little influence, and the system has a strong anti-interference ability, which provides a basis for the system to realize large-scale measurement; an adjustable attenuator is added to the feedback external cavity, and the control system works at a strong feedback level Next, due to the strong light feedback effect, when the measured object moves a quarter of the light wavelength along the laser axis, the laser intensity fluctuates periodically, and four polarization states appear in one cycle: o light area, o light Coexistence area with e-light, e-light area and no-light area, each area corresponds to one-sixteenth light wavelength displacement of the object, the resolution of the system is eight times higher than that of the traditional feedback measurement system, and when the measured object moves in the direction When changing, the order in which the four polarization state regions appear in each cycle changes, thereby easily realizing the discrimination of the displacement direction of the object.

The experimental device (example one) of the present invention is shown in Figure 1. In Fig. 1, 4 is the main beam output mirror of the laser, its inner surface is coated with a high reflection film, and the reflectivity is 99.0%, the upper part of the outer surface is coated with a high reflection film, and the reflectivity is 99.0%, and the lower part is coated with a Permeable membrane, as shown in Figure 2; 5 is the laser gain tube, which is filled with a mixed gas of helium and neon. The helium gas uses He ³ isotopes, and the neon gas uses Ne ²⁰ and Ne ²² isotopes ^. : Ne ²⁰ : Ne ²² = 7: 0.5: 0.5; 7 is a quartz crystal wedge, and its two sides are all coated with anti-reflection coating, as shown in Figure 4; 8 is a concave mirror, which is the tail beam output mirror of the laser; 6 is two A permanent magnet, which applies a uniform transverse magnetic field parallel or perpendicular to the polarization direction of any polarized light produced by the above-mentioned lasers to the entire gain tube; the inner surface of 4 and 8 form a laser resonator cavity, 4, 5, 6, 7 and 8 together form a Zeeman birefringent dual-frequency laser; 1 is a hollow corner cube, which is composed of three mutually perpendicular glass plates coated with a high-reflection film with a reflectivity of about 99.0%, as shown in Figure 3; 2 is An absorbing adjustable attenuator; 3 is a beam collimating and expanding device, which is composed of a group of lenses; the outer surface of 4 and the inner surfaces of 1 and 4 together form a feedback folding cavity; 1, 2, 3 and 4 together form a Collimated strong light feedback folding cavity; 9 is a polarizing beam splitter; 10 and 11 are two photodetectors; 9, 10 and 11 constitute a signal receiving device, and the detected signal is input to the amplification and filtering circuit 12, and the After the signal is amplified and filtered, it is input to the logic direction determination and counting circuit 13, and the size and direction of the motion displacement of the measured object are obtained through signal processing, which is displayed by the display device 15; 14 is the system base; 16 is the glass cover, which reduces external Environmental interference; 17 is the external measured object.

Principle of the present invention is as follows:

In the case of optical feedback for a single-mode He-Ne laser, the gain variation Δg per unit length is:

$\mathrm{<span\; class="notranslate">\; \&Delta;g</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; g</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="0"\; label="g"\; filenames="A20061008884600142.png"\; state="\{\{state\}\}">g</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; L</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; \&pi;vl</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In the formula, g is the linear gain per unit length when there is optical feedback, g ₀ is the linear gain per unit length when there is no optical feedback, α=t ₂ r ₃ ξ/r ₂ , where r ₂ and t ₂ are respectively is the reflection coefficient and projection coefficient of the inner surface of the main output mirror of the laser, _r2 is the reflection coefficient of part of the outer surface of the main output mirror of the laser, ξ is the attenuation coefficient of the adjustable attenuator, v is the laser frequency, c is the vacuum The speed of light, L is the length of the laser cavity, and l is the change of the length of the laser feedback cavity. δ is the fixed phase difference caused by light feedback in the external cavity.

For a He-Ne dual-frequency laser, the two orthogonal polarization modes share the same gain in the laser cavity. Due to the mode competition effect between the two modes, the linear gain variation within the unit length of the two modes can be obtained as follows:

$\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; g</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; g</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; L</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; \&pi;vl</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ,</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Δg _o and Δg _e are the linear gain variation within the unit length of o light and e light, respectively.

Since the dual-frequency laser used in the system is a Zeeman birefringent dual-frequency laser, its frequency difference is much smaller than the uniform broadening of the laser medium spectrum (about 200MHz), and the phase difference Δφ between o and e light is determined by the mode competition. The result is Δφ≈π/2.

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

I=I ₀ -kΔg, (3)

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

Then under the condition of optical feedback, the light intensity of the two orthogonal polarization modes of the laser is:

I _o ＝I _0o +αK/2L·cos(4πvl/c+δ)

I _e =I _0e -αK/2L·cos(4πvl/c +δ+Δφ), (4)

In the formula, I _o and I _e are the light intensities of o light and e light when light is fed back, and I _0o and I _0e are the initial light intensities of o light and e light when there is no light feedback.

For the folded feedback system, if the displacement of the measured object changes as Δl, then the length of the feedback cavity changes as l=2Δl. Then the phase change caused by the displacement of the external object is:

Δδ＝4πvl/c＝2π·2Δl/(λ/2)＝2π·Δl/(λ/4). (5)

It can be seen from formula (5) that the laser intensity fluctuates for one cycle when the displacement of the object changes by a quarter of the wavelength.

The system works at the level of strong feedback, ξ is larger, so α is larger, and the modulation depth M of laser intensity is also larger. M can be expressed as:

M=αK/2L. (6)

Considering that the laser intensity has no negative value, the expressions of o and e light intensity in the collimated strong light feedback folding system are:

$\left\{\begin{array}{cc}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&alpha;K</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 8</span>}\mathrm{<span\; class="notranslate">\; \&pi;v\&Delta;l</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ,</span>& <span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; \&Greater\; Equal;</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span>\\ {\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; ,</span>& <span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; <</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span>\end{array}\right.<span\; class="notranslate">\; ,</span>$

$\left\{\begin{array}{cc}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&alpha;K</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 8</span>}\mathrm{<span\; class="notranslate">\; \&pi;v\&Delta;l</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; c</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ,</span>& <span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; \&Greater\; Equal;</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span>\\ {\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; e</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; ,</span>& <span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; <</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span>\end{array}\right.<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

When the frequency difference Δv of the laser is 8.86MHz, the length L of the inner cavity is 190mm, the length l of the external feedback cavity is 380mm, and the total feedback level of the system is 84.8%, as the measured object moves along the laser axis, the The strong light folding feedback effect makes the light intensity change curve of two orthogonally polarized lights as shown in FIG. 8 . The dotted line is the light intensity curve of o light feedback, the solid dotted line is the light intensity curve of e light feedback, and the triangular wave is the driving voltage curve of piezoelectric ceramics driving the measured object.

From Figure 8, we can see that the light intensity of o and e lights changes periodically, and the light intensity modulation depth is very large. When the object moves in opposite directions, the light intensity modulation curves of o and e lights are symmetrical. In each period, o and e light alternately oscillate, and there are no light areas, the width of light output and the width of no light are almost equal, and there is a phase difference of about π/2 between o and e light, so there are four in each period Two states: one is only the e light oscillates, and the o light is extinguished at this time, which is called the e light zone; the other is the co-existence zone of the o light and the e light, and both the o light and the e light can oscillate; the third is the o light zone, only the o The light oscillates, and the e light is extinguished at this time; the fourth is the no-light area, and the o and e lights do not oscillate. o, e A period of the light intensity modulation curve corresponds to a quarter of the light wavelength displacement of the measured object, and each zone corresponds to a 16th light wavelength displacement of the measured object. For a 632.8nm He-Ne laser, The resolution of the system is 39.55nm, which is eight times the resolution of common feedback systems.

According to formula (7), the simulation analysis is carried out, and the feedback light intensity curve of the two orthogonally polarized lights in the feedback process is obtained through numerical simulation, as shown in Fig. 9 . The experimental results in Figure 8 are in good agreement with the numerical simulation results in Figure 9.

Fig. 10 is a schematic diagram of the system identifying the displacement direction of the measured object. In one cycle, the o and e light feedback curves are divided into four parts, AB is the no light zone, BC is the e light zone, CD is the o and e light coexistence zone, DE is the o light zone, EF is the no light zone, FG is e light area. When the piezoelectric ceramic 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 zones is no light zone, e light zone, o and e light coexistence zone, o light zone, no light area; when the piezoelectric ceramic voltage decreases, that is, when the measured object moves to the direction of increasing the length of the feedback cavity, the order of the four areas is no light area, o light area, o, e light coexistence area, e light area, No light area; when the measured object has different displacement directions, the four areas appear in different orders, and the identification of the object displacement direction can be easily realized through signal processing.

Figure 11 is the feedback light intensity curve of the system when the measured object produces different displacements, where (a) is the feedback light intensity curve when the displacement is 10mm, and (b) is the curve when the displacement is 20mm. It can be seen from Figure 11 that when the object moves in a large range, the o and e light intensity curves are almost unchanged, and the curves can still be divided into four areas within one cycle, realizing the recognition of the displacement direction of the measured object and the improvement of resolution . He-Ne lasers have good coherence. In principle, the maximum displacement range that the displacement measurement system designed in the present invention can measure is half of the coherence length of the laser. Experiments show that the measurement range of the displacement measurement system is at least 20mm. The device has a very large measurement range while having high resolution.

The schematic structural diagram of the second example of the present invention is shown in FIG. 6 . The structure shown in Fig. 6 is basically the same as that shown in Fig. 1, and seventeen elements from 1 to 17 are the same as those in Fig. 1 except for 7, which will not be repeated here. 7 is a window, both sides are coated with anti-reflection film, fixed on one side of the gain tube; 18 is a quartz crystal, placed between the window 7 and the tail beam output mirror 8, used to generate frequency split. 4, 5, 6, 7, 8 and 18 together constitute a Zeeman birefringent dual-frequency laser. The resolution of the system is still one-sixteenth of the light wavelength, which can identify the displacement direction of the measured object, and has a large measurement range and strong anti-interference ability.

The schematic structural diagram of the second example of the present invention is shown in FIG. 7 . The structure shown in FIG. 7 is basically the same as that shown in FIG. 1 , and seventeen elements from 1 to 17 are the same as those in FIG. 1 except for 7, which will not be repeated here. 7 is a window, both sides are coated with anti-reflection film, fixed on one side of the gain tube; 18 is a stress applying device, which applies a stress to the window 7 along the axis direction of the vertical laser gain tube, due to the double Refraction effect, one frequency can be divided into two frequencies. 4, 5, 6, 7, 8 and 18 together constitute a Zeeman birefringent dual-frequency laser.

The present invention designs a displacement measurement system based on the strong light folding feedback effect of beam expansion in Zeeman birefringence dual-frequency lasers. The device uses Zeeman birefringence dual-frequency lasers, beam collimation expanders, hollow corner cubes, An adjustable attenuator and a feedback mirror with high reflectivity constitute a beam-expanding strong light folding feedback system. The hollow corner cube prism is fixed on the measured object. When the measured object moves a quarter of the light wavelength along the laser axis, the laser intensity fluctuates periodically, and there are four polarization state regions in one cycle: o optical region , o-light and e-light coexistence area, e-light area and no-light area, each area corresponds to the displacement of one sixteenth of the light wavelength of the object, that is, 39.55nm, and the displacement of the measured object can be obtained by detecting the number of polarization state areas ; When the moving direction of the measured object changes, the order in which the four polarization regions appear in each period changes, so that the displacement direction of the object can be judged. The device has the characteristics of high precision, large measuring range, easy direction judgment, strong anti-interference ability and high cost performance.

Claims (3)

1, expand the folding displacement measuring system of high light of bundle, it is characterized in that described system contains: expand the folding feedback device of high light of bundle, comprising:

The Zeeman birefringence double-frequency laser, the output frequency difference is less than the pairwise orthogonal linear polarization frequency of 40MHz, and this laser instrument contains:

Laser gain pipe, in be filled with helium, neon mixed gas;

The quartz crystal wedge is fixed in a side of above-mentioned laser gain pipe; This quartz crystal wedge two sides all is coated with anti-reflection film;

Laser resonant cavity comprises main beam outgoing mirror and tail light beam outgoing mirror; The main beam outgoing mirror is fixed on the opposite side of above-mentioned laser gain pipe, and this main beam outgoing mirror inside surface is coated with the strong reflection film, and the top of outside surface also is coated with the strong reflection film, and the lower part of outside surface is coated with anti-reflection film; Tail light beam outgoing mirror is positioned at the outside of above-mentioned quartz crystal wedge, and this outgoing mirror inside surface is coated with the strong reflection film;

The transverse magnetic field generator is made of two blocks of permanent magnets that are parallel to described laser gain pipe axis up and down, and whole gain tube is in the magnetic field, the polarization direction of magnetic direction any one linearly polarized light parallel or that produce perpendicular to above-mentioned laser instrument;

The laser feedback exocoel is made of the top of the outside surface of the inside surface of above-mentioned main beam outgoing mirror and above-mentioned main beam outgoing mirror, and this main beam outgoing mirror is not only done the laser endoscope but also make exocoel feedback mirror;

Feedback exocoel folders, it is a hollow prism of corner cube, constitute by three blocks of mutually perpendicular glass plates that are coated with the strong reflection film, place the feedback light path of above-mentioned laser feedback exocoel, the feedback light of laser instrument master output terminal output is successively by behind described three blocks of glass plates, by the top reflected back laser feedback exocoel of the outside surface of described main beam outgoing mirror;

The beam collimation extender is made up of one group of lens that are parallel to each other perpendicular to the feedback light of laser instrument master output terminal output, places the main output terminal of above-mentioned laser feedback light path, and the feedback light that laser instrument master output terminal is exported collimates and expands bundle;

The laser feedback horizontal controller is an absorptive-type adjustable attenuator, places a side of above-mentioned beam collimation extender, makes system works under strong feedback level; Acquisition of signal and processing section comprise:

Polarization spectroscope is positioned at the outside of above-mentioned tail light beam outgoing mirror, is used for the light of two frequencies of the polarization state quadrature of tail light beam outgoing mirror output is spatially separated;

Photodetector, totally two, all be positioned at the outside of above-mentioned polarization spectroscope, be used for surveying respectively the light intensity of two frequencies of light of tail light beam outgoing mirror output;

Amplify and filtering circuit, input end links to each other with the output terminal of above-mentioned two photodetectors respectively, and the signal that above-mentioned photodetector is detected amplifies and filtering;

Logic declare to and counting circuit, successively by two voltage comparators, two monostable triggers, forwards with oppositely totally two rejection gates and a up-down counter series connection constitute; Two voltage comparator input ends link to each other with the output terminal of above-mentioned amplification and filtering circuit respectively, two monostable trigger input ends link to each other with the output terminal of above-mentioned two voltage comparators respectively, the saltus step of the every output primary voltage of voltage comparator, monostable trigger just generates a pulse signal, is input in two rejection gate elements; When system's forward moves, forward rejection gate output direct impulse, oppositely rejection gate no-output; When system reverse moved, oppositely rejection gate was exported reverse impulse, forward rejection gate no-output; Forward and reverse impulse signal draw the size and Orientation of ohject displacement by the positive and negative plus-minus end of being delivered to up-down counter respectively;

Display circuit, its input end is declared to linking to each other with the output terminal of counting circuit with above-mentioned logic, and numeral shows the displacement size and Orientation of testee along the laser axis direction;

Base portion contains fixed support, is fixedly linked with above-mentioned laser gain pipe, transverse magnetic field generator, beam collimation extender, laser feedback horizontal controller, laser instrument tail light beam outgoing mirror and polarization spectroscope respectively;

2, the quartz crystal wedge narrated of claim 1 is replaced by glass window and a quartz crystal that a slice two sides all is coated with anti-reflection film, this glass window is fixed on the end of laser gain pipe near tail light beam outgoing mirror, this quartz crystal is fixed on the base, and places between glass window and the laser instrument tail light beam outgoing mirror;

3, the quartz crystal wedge narrated of claim 1 is by all being coated with the glass window of anti-reflection film and of being clipped on this window answers forcer to replace by a slice two sides, and this glass window is fixed on the end of laser gain pipe near tail light beam outgoing mirror.

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