CN108692663B - Phase modulation type orthogonal polarization laser feedback grating interferometer and its measurement method - Google Patents
Phase modulation type orthogonal polarization laser feedback grating interferometer and its measurement method Download PDFInfo
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Abstract
本发明涉及一种用于二维测量的相位调制型正交偏振激光反馈光栅干涉仪及其测量方法，测量原理基于光栅衍射、光学多普勒效应、Lamb半经典理论和时域正交解调原理。双折射双频氦氖激光器输出的正交偏振光垂直入射至偏振分光棱镜被分成两束不同偏振方向的线偏振光，这两束偏振光分别经过两个不同的电光调制器后分别被反射镜以±1级利特罗入射角入射到反射式衍射光栅上。衍射光分别沿各自的入射光返回激光腔内与腔内光发生自混合干涉。激光器后向输出光经偏振片后只保留单模光，并被光电探测器接受，光电探测器输出信号输出至数据处理模块进行数据处理，得到待测目标的二维位移。本发明具有结构简单，测量范围大、测量分辨率高等优点。
The invention relates to a phase modulation type orthogonal polarization laser feedback grating interferometer for two-dimensional measurement and a measurement method thereof. The measurement principle is based on grating diffraction, optical Doppler effect, Lamb semi-classical theory and time domain quadrature demodulation principle. The orthogonally polarized light output by the birefringent double-frequency HeNe laser is vertically incident on the polarizing beam splitter prism and is divided into two linearly polarized lights with different polarization directions. Incident on a reflective diffraction grating with a ±1st order Littrow incident angle. The diffracted light returns to the laser cavity along the respective incident light, and self-mixing interference occurs with the light in the cavity. The backward output light of the laser only retains single-mode light after passing through the polarizer, and is accepted by the photodetector. The output signal of the photodetector is output to the data processing module for data processing to obtain the two-dimensional displacement of the target to be measured. The invention has the advantages of simple structure, large measurement range and high measurement resolution.
Description
Technical Field
The invention belongs to the technical field of precision displacement measurement, and particularly relates to a phase modulation type orthogonal polarization laser feedback grating interferometer for two-dimensional measurement and a measurement method thereof.
Background
Nano-measurement is a key technology for the development of advanced manufacturing industry and is also a lead and foundation of the whole nano-technology field. With the development of ultra-precision machining and ultra-fine machining technologies, the demand for real-time high-precision two-dimensional positioning systems is rapidly increasing. Laser interferometers and grating interferometers are widely used for high-precision displacement measurement due to their advantages of non-contact, high resolution, wide dynamic measurement range and the like. Typically, laser interferometry is used to measure out-of-plane displacements, while grating interferometry is used to measure in-plane displacements.
The solutions that are currently available to perform two-dimensional measurements are mainly achieved by using two sets of interferometers, using two-dimensional gratings and beam splitting techniques, or replacing the mirrors of the interferometers with reflection gratings. The use of two sets of interferometers is the most straightforward method, but its simultaneity cannot be guaranteed. The grating interferometer based on the two-dimensional diffraction grating shows good performance in the aspect of high-precision two-dimensional positioning, but the system arranges the grating in a reflecting plane and utilizes the two-dimensional grating to realize the measurement of two-dimensional displacement in the plane, and the manufacturing cost of the two-dimensional grating is very expensive. In some specific application scenarios, such as probe-based near-field microscopy and optical imaging, a quasi-confocal optical path heterodyne grating interferometer can also achieve high-precision two-dimensional in-plane measurement. In recent years, much attention has been paid to interferometers that use a heterodyne grating instead of a mirror, which consists of a reference grating and a measurement grating that can measure both in-plane and out-of-plane displacements. Thereafter, many in-plane and out-of-plane displacement measurement systems based on this approach were developed. However, such systems generally face two problems: when the target moves a distance in an out-of-plane direction, the optical path of these systems will also change, causing a deviation between the detected light and the photodetector. Therefore, the measurement of out-of-plane displacement is typically used to compensate for the in-plane displacement measurement; and these optical path systems are often very complex and difficult to adjust.
Disclosure of Invention
The technical problem to be solved by the invention is as follows:
the invention provides a phase modulation type orthogonal polarization laser feedback grating interferometer and a measuring method thereof, in order to enable a two-dimensional micro-displacement measuring device to have wide range and high resolution and be suitable for industrial field measurement.
The invention adopts the following technical scheme for solving the technical problems:
a phase modulation type orthogonal polarization laser feedback grating interferometer is provided, which comprises: the device comprises a double-refraction double-frequency helium-neon laser, a Wollaston prism, a first electro-optic modulator, a second electro-optic modulator, an electro-optic modulator driver, a first plane reflector, a second plane reflector, a reflection type diffraction grating, a polaroid, a photoelectric detector and a signal processing module A.
The birefringent double-frequency He-Ne laser emits cross-polarized laser, and is divided into two laser beams with different polarization directions by the Wollaston prism, wherein the horizontally polarized laser passes through the first electro-optical modulator, is incident to the reflective diffraction grating by the first plane mirror at a littrow incidence angle of +1 order, and the vertically polarized laser passes through the second electro-optical modulator, is incident to the reflective diffraction grating by the second plane mirror at a littrow incidence angle of-1 order; the electro-optical modulator driver outputs two voltage signals with different frequencies to respectively drive the two electro-optical modulators, and outputs reference signals corresponding to the two voltage signals to the signal processing module A; the laser is incident to the diffraction light generated by the reflection type diffraction grating and returns to the double refraction double-frequency helium-neon laser along an incident path, and the laser feedback interference is generated between the laser and the light in the cavity; the polaroid is arranged on a backward output light path of the birefringent double-frequency He-Ne laser, the photoelectric detector is arranged behind the polaroid, and the photoelectric detector outputs the light to the signal processing module A;
the signal processing module A comprises: the device comprises an operational amplifier, a first band-pass filter, a second band-pass filter, a third band-pass filter, a fourth band-pass filter, a data acquisition card and a computer; the detection signal is input to the operational amplifier, the operational amplifier outputs an amplified signal to the first band-pass filter, the fourth band-pass filter outputs four filtered signals to the data acquisition card at the same time, the data acquisition card performs analog-to-digital conversion and then inputs the signals into the computer, and the signals are processed by the computer to obtain the displacement to be detected.
In the phase modulation type orthogonal polarization laser feedback grating interferometer, the birefringent double-frequency he-ne laser outputs double longitudinal mode orthogonal polarization laser, and mode competition exists between the two modes.
In the phase modulation type orthogonal polarization laser feedback grating interferometer, the Wollaston prism uses α -BBO, calcite or yttrium vanadate as a substrate, and the separation angle of an emergent beam is between 17 and 23 degrees.
In the phase modulation type orthogonal polarization laser feedback grating interferometer, the main axis direction of the first electro-optical modulator and the main axis direction of the second electro-optical modulator are consistent with the polarization direction of the passing laser; the electro-optical modulator is used for carrying out pure phase modulation on the passing laser.
The phase modulation type orthogonal polarization laser feedback grating interferometer is characterized in that the first electro-optical modulator and the second electro-optical modulator are used for performing sinusoidal phase modulation on passing laser, the modulation amplitude is pi/2, and the modulation initial phase is 0; the ratio of the modulation frequencies of the first electro-optical modulator and the second electro-optical modulator is 3: 7.
In the phase modulation type orthogonal polarization laser feedback grating interferometer, the first electro-optical modulator and the second electro-optical modulator are waveguide-shaped electro-optical crystals.
In the phase modulation type orthogonal polarization laser feedback grating interferometer, the reflective diffraction grating is a holographic grating made of Borofloat glass.
In the phase modulation type cross polarization laser feedback grating interferometer, the polarization direction of the polarizer is adjusted to allow only o light or e light of the birefringent double frequency he-ne laser to pass through.
The phase modulation type orthogonal polarization laser feedback grating interferometer as described above, further, a center frequency of the first band pass filter is equal to a modulation frequency of the first electro-optical modulator, a center frequency of the second band pass filter is equal to twice the modulation frequency of the first electro-optical modulator, a center frequency of the third band pass filter is equal to the modulation frequency of the second electro-optical modulator, and a center frequency of the fourth band pass filter is equal to twice the modulation frequency of the second electro-optical modulator; the first to fourth band-pass filters have the same bandwidth and reach the maximum under the premise that the pass bands are not overlapped.
The invention also provides a measuring method based on the phase modulation type orthogonal polarization laser feedback grating interferometer, wherein the phase demodulation adopts a time domain orthogonal demodulation technology, and the method comprises the following specific steps:
(1) processing the filtered signals output by the first to fourth band-pass filters by using a real-time normalization algorithm;
(2) respectively removing respective carrier waves of the processed filtering signals to obtain two groups of sine components and cosine components of the phase to be measured;
(3) demodulating two groups of phases to be detected;
(4) and measuring the target displacement in real time according to the linear relation between the two groups of phases to be measured and the two-dimensional displacement of the reflection type diffraction grating.
Compared with the prior art, the invention adopting the technical scheme has the following technical effects:
1) the invention adopts the laser feedback grating interference principle, does not need auxiliary elements such as a reference grating of the traditional grating interferometer, and the like, two beams of orthogonally polarized light are simultaneously fed back to the birefringent double-frequency He-Ne laser to generate interference, and the signal detection can be realized only by one photoelectric detector, thereby greatly simplifying the structure of an optical path system and facilitating the adjustment of the optical path.
2) Compared with the existing grating interference technology for realizing two-dimensional displacement measurement, the invention utilizes the advantages of the littrow structure, the optical path structure cannot be changed when the grating generates out-of-plane displacement, the out-of-plane wide-range displacement measurement is realized, the number of devices is reduced, and the system complexity is reduced.
3) The invention adopts the electro-optical modulator to carry out pure phase modulation on the diffracted light, has high modulation precision and wide modulation bandwidth, realizes phase demodulation by a time domain orthogonal demodulation technology, has simple demodulation method algorithm, is insensitive to sampling error, and can greatly improve the measurement resolution of the displacement measurement device.
4) The invention forms a new two-dimensional micro-displacement measuring device with wide range and high resolution and is suitable for industrial field measurement, and has important practical significance for further promoting the development of advanced manufacturing technology.
Drawings
Fig. 1 is a schematic structural view of the present invention.
Fig. 2 is a time domain quadrature demodulation diagram of the present invention.
Detailed Description
The technical scheme of the invention is further explained in detail by combining the attached drawings:
it will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The laser feedback interference technology is a novel interference metering technology with high application value, when output light of a laser is reflected or scattered by an external object, part of the light returns to the inside of a laser resonant cavity and is mixed with light beams in the cavity to cause the change of the output light intensity of the laser, and the precise measurement of physical quantities such as speed, displacement, vibration, distance and the like is realized. The system has the obvious advantages of simple and compact structure, self-collimation and capability of working on a rough scattering surface, solves the problems of complexity, sensitivity to collimation and the like of the traditional interferometric technology system, and can replace the traditional laser interferometer in many occasions.
The working principle of the phase modulation type orthogonal polarization laser feedback grating interferometer of the invention is explained with reference to fig. 1. As shown in fig. 1, orthogonally polarized light output from a birefringent double-frequency he-ne laser 1 is perpendicularly incident on a wollaston prism 2 and is split into o light and e light. The first electro-optical modulator 3 is arranged on an o-light path and used for carrying out pure phase modulation on the o-light, and the modulation function MoComprises the following steps:
Mo＝(π/2)sin(2πfomt) (1)
wherein f isomThe modulation frequency of the first electro-optic crystal 3; the second electro-optical modulator 4 is arranged on the e light path and is used for carrying out pure phase modulation on the e light, and the modulation function MeComprises the following steps:
Me＝(π/2)sin(2πfemt) (2)
wherein f isemThe modulation frequency of the second electro-optical crystal 4. Modulating frequency f to accurately reflect the motion state of the reflective diffraction gratingomAnd femMuch greater than the rate at which the reflective diffraction grating is displaced,specifically, it is required to satisfy:
fom/3＝fem/7>>vxm/d+vzm(2/λcosθ-tanθ/d) (3)
in the formula vxmIs the maximum movement speed, v, of the reflective diffraction grating (8) in the in-plane directionzmThe maximum movement speed of the reflective diffraction grating 8 in the out-of-plane direction is d, which is a grating constant, λ is the center wavelength of the birefringent dual-frequency he-ne laser 1, and θ is the littrow incident angle.
The split o and e light is reflected by the first and second planar mirrors 6 and 7, respectively, and is incident on the reflective diffraction grating 8 at +1 and-1 littrow incident angles θ, respectively. The littrow structure enables the diffracted light to return to the cavity of the birefringent double-frequency He-Ne laser 1 along an incident light path to generate laser feedback interference with light in the cavity.
When the reflective diffraction grating 8 moves by Δ x in the x direction in the figure, the feedback light phase change of o light caused by the displacement of the reflective diffraction grating 8 is:
where d is the grating constant. The feedback light phase change of the e light is:
when the reflective diffraction grating 8 moves by Δ z in the z direction in the figure, the feedback light phase change of o light and e light caused by the displacement of the reflective diffraction grating 8 is:
where θ is the littrow angle of incidence. Because the o light and the e light respectively pass through the electro-optical modulator twice in the external cavity, the phase changes of the feedback light of the o light and the e light caused by the electro-optical modulator are respectively as follows:
the total variation of the feedback light phase of the o light and the e light is respectively as follows:
according to Lamb semi-classical theory, the output light field E of the double longitudinal mode lasero/e(t) can be expressed as:
Eo＝E0+(αoβe-αeθoe)/(βoβe-θoeθeo) (11)
Ee＝E0+(αeβo-αoθeo)/(βoβe-θoeθeo) (12)
in the formula, subscripts o and e represent a variable corresponding to o light and a variable corresponding to e light respectively, and the meanings of the variables are as follows: e0At an initial light intensity of αo，αeLinear gain coefficients for o-light and e-light, βo，βeIs the intensity self-saturation constant of o and e lightoe，θeoIs the mutual saturation constant of o light and e light, wherein the linear gain coefficient αo，αeComprises the following steps:
αo＝α’o-fo/Qo(13)
αe＝α’e-fe/Qe(14)
wherein the meanings of the variables are α'o，α’eSmall signal linear gain for o-and e-light，fo/e，fo/eLaser frequency, Q, of o and e lighto，QeQuality factor of the resonant cavity for o-light and e-light. Wherein the quality factor Qo，QeCan be expressed as:
Qo＝4πL/[λ(2-R1-Ro)](15)
Qe＝4πL/[λ(2-R1-Re)](16)
in the formula, the variables have the following meanings: l is the laser cavity length, R1Is the reflectivity, R, of the left end resonator mirror of the birefringent double-frequency He-Ne laser 1 in FIG. 1o，ReThe dynamic reflectivity of o light and e light is shown by the influence of the external cavity formed by the right resonator mirror of the double-refraction double-frequency He-Ne laser 1 and the reflection type diffraction grating 8. Dynamic reflectivity R of reflective diffraction grating 8 when it is displaced in xz planeo，ReCan be expressed as:
in the formula, the variables have the following meanings: r2Is the reflectivity of the right-hand resonator mirror of the birefringent double-frequency He-Ne laser 1 in FIG. 1, η is the first-order diffraction efficiency of the reflective diffraction grating 8, lo，leIs the outer lumen length. Substituting expressions (13) to (18) into expressions (11) to (12) to obtain an approximate solution of laser feedback interference:
in the formula, A and B are simplified constant coefficients. By developing equation (19), the following can be obtained:
in the formula Jn(π) represents a value of a Bessel function of order n of π. After the left output light of the birefringent double-frequency he-ne laser 1 passes through the polaroid 9, one mode of o light and e light is filtered, and the photoelectric detector 10 detects the output light intensity of the other mode of the birefringent double-frequency he-ne laser 1 and inputs the output light intensity into the signal processing module A. Here, the displacement measurement method of the phase modulation type orthogonal polarization laser feedback grating interferometer according to the present invention is described with reference to fig. 2 by taking the output o light as an example. The optical power fluctuation caused by the feedback of the e-beam is also reflected in the o-beam due to the mode competition effect. The photodetector 10 converts the detected light intensity signal into an electrical signal and outputs the electrical signal to the operational amplifier 11, and the amplified signal is equally divided into four parts and simultaneously input to four band-pass filters 12-15, wherein the center frequency of the first band-pass filter 12 is the modulation frequency f of the first electro-optical modulator 3omThe second band-pass filter 13 has a center frequency of 2fomThe third band-pass filter 14 has a center frequency at the modulation frequency f of the second electro-optical modulator 4emThe fourth bandpass filter 15 has a center frequency of 2fem. The filtered signals are sequentially:
in the formula S1-S4Respectively representing the output signals of the first to fourth bandpass filters 12-15, which are coupled to two modulation frequencies f generated by the electro-optical modulation drive 5om，femThe corresponding reference signals are input into the signal acquisition card 16 together and processed by the computer 17. The reference signal output by the electro-optical modulation driver 5 is converted into 4 carriers by the computer 17, which are respectively: c1＝cos(2πfomt)，C2＝cos(4πfomt)，C3＝cos(2πfemt)，C4＝cos(4πfemt). Filtered signal S1-S4Divided by carrier C respectively1-C4Then, through normalization, the following results can be obtained:
the grating displacement results in a phase change of the o-light and e-light, respectively
And
can be expressed as:
the phase calculated by the arctangent function is wrapped between [ -pi, pi ], and after unwrapping operation, the obtained grating displacement is:
the invention provides a phase modulation type orthogonal polarization laser feedback grating interferometer and a measuring method, which are used for measuring the in-plane and out-of-plane displacement of a target in real time.
The foregoing is only a partial embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
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