TWI437208B - Length measuring device - Google Patents

Description

Length measuring device

The present invention relates to a length measuring device, and more particularly to a length measuring device using a Fabry-Perot interferometer principle in combination with a line array detector.

In 1897, C. Fabry and A. Perot invented the multi-beam interferometer, which is now the Fabry-Perot interferometer. Because the interference fringes produced by the Fabry-Perot interferometer are very sharp and concentrated, the fringe contrast is high, and the exact position can be accurately measured. It is the most accurate interferometer at that time, and it is still an indispensable effective measuring tool until today.

The Fabry-Perot interferometer has excellent measurement accuracy and environmental interference resistance. However, the interference fringes of the Fabry-Perot interferometer have the characteristics of high sharpness and discontinuity, so it is impossible to solve the problem by the traditional interference signal processing method. A main cause of limited range. The stability of the general interferometer is poor, and the signal changes in a sinusoidal shape, and the position uncertainty is high. The traditional signal processing method is mostly based on the time-varying signal of the sine wave. Therefore, how to eliminate the DC direct level and ensure the signal Orthogonality and how to maintain constant amplitude become the three major problems affecting accuracy.

Taiwan Patent Publication No. 201030309 discloses a high-precision displacement sensing interferometer, which is a Fabry-Perot interferometer combined with a position sensitive sensor, which can perform high-precision displacement measurement. However, the calculation of the displacement measurement of the above-mentioned Taiwan public patent is very complicated, and it cannot solve the problem of measuring a large distance and having high precision.

As shown in FIG. 1, the conventional Fabry-Perot interferometer is composed of two plate-shaped fixed mirrors 11 and moving mirrors 12 of a specific reflectance. The surfaces of the fixed mirror 11 and the moving mirror 12 must be very flat and have a highly reflective coating on one side. Since the fixed mirror 11 and the moving mirror 12 are separated by a distance d, they are close to each other. When a light beam E is incident between the two reflecting surfaces, the light beam is reflected back and forth in the resonant cavity 13 a plurality of times, and the partial light beams E1, E3, E5, and E7 are transmitted through the moving mirror 12, and the partial light beams E2, E4, and E6 are The fixed mirror 11 is transmitted. Since there is an optical path difference between the transmitted light, if these transmitted beams E1, E3, E5, and E7 are superimposed, interference fringes are generated; in order to generate spatially distributed interference fringes, the fixed mirror 11 and the moving mirror are often used. 12 One of the two is tilted by a slight angle.

According to the geometric relationship, the optical path difference (ΔL) and phase difference (δ) between each adjacent two reflected or transmitted light can be derived: ΔL=2d/cos α; the phase difference between adjacent transmitted beams It is: δ=4 π d/λ cos α; where: α is the incident angle, d is the distance between the fixed mirror 11 and the moving mirror 12, λ is the wavelength of the incident light beam E, and n is the fixed mirror 11 and the moving mirror 12 The refractive index between the media.

The total amplitude of the transmitted beam and the transmitted light beam are: _t = E ₀ (1-R) [1 + Re ^{i δ} + (Re ^{i δ} ) ² +...] = E ₀ (1-R) / (1-Re ^{i δ} ); I _t =| _t | ² =I ₀ (1-R) ² /(1+R ² -2Rcos δ); I ₀ =(E ₀ ) ² .

The distance between two peaks of the Fabry-Perot interference fringe (L) corresponds to the length of λ/2. When the distance d between the fixed mirror 11 and the moving mirror 12 is changed, the relative position of the interference fringes also changes, so that the moving mirror 12 is moved for a distance and the change of the interference fringe position is observed ΔL, and the displacement amount can be calculated. x.

The position of the initial interference fringe is recorded in the Fabry-Perot interference fringe experiment with a laser beam with a wavelength of 657.2 nm (nano), as shown in Fig. 2; the position of the interference fringe is recorded after the displacement of the moving mirror, as shown in the figure As shown in Fig. 3, the difference in peak position before and after is compared, and the displacement is calculated by using the relationship between the pitches of the two peaks of the interference fringes corresponding to the length of λ/2.

Δx = (ΔL/L) × (λ/2).

The absolute distance measurement method can be roughly divided into a linear frequency modulation method and a decimal weight method. The linear frequency modulation is modulated for time, and the signal processing portion of the software and hardware is more complicated. Fractional weighting is to use the fractional part ε of non-integer variation of interference fringes to solve the length to be measured, which is mainly summarized as follows: (1) using multiple sets of single wavelengths to combine different lengths of synthesized wavelengths; (2) using different synthetic wavelengths, Solve the measured length one by one.

If the measured distance L is measured with two wavelengths, respectively: L = (λ ₁ /2) (m ₁ + ε ₁ )

L = (λ ₂ /2) (m ₂ + ε ₂ )

Wherein m ₁ and m ₂ respectively corresponding to wavelengths λ ₁ and λ ₂ integral multiple of the interference fringes, ε _1, ε ₁ corresponding to the fractional part of the interference fringes, the above two simultaneous solution of formula is obtained: L = (1 /2) × [(λ ₁ × λ ₂ ) / (λ _{1 -} λ ₂ )] × [(m _{2 -} m ₁ )(ε ₂ - ε ₁ )]; The above formula can be simplified to: L = ( λ _s /2) × (m ₂ + ε _s ); λ _s = (λ ₁ × λ ₂ ) / (λ _{1 -} λ ₂ ); where: m _s = m _{2 -} m ₁ , ε _s = ε ₂ - ε ₁ ; m _s =int[2L _c /λ _s 〕+1, ε _s <0; m _s =int[2L _c /λ _s ], ε _s >0; Lc is the initial value. Therefore, the measured distance L can be calculated from λ ₁ , λ ₂ , ε ₁ , ε ₁ and Lc. This is a well-known technique.

The present invention has been proposed in order to further improve the measuring device of the known Fabry-Perot interferometer.

The main object of the present invention is to provide a length measuring device which uses a dual-wavelength synthetic Fabry-Perot interferometer combined with the structure of two line array detectors to measure the length of the object to be tested, thereby improving the Fabry-Perot interferometer. Precision measurement capability.

Another object of the present invention is to provide a length measuring device that utilizes a measurement architecture of a Fabry-Perot interferometer, uses a temperature control method to ensure the stability of the laser wavelength, and expands the laser beam by a collimator. After measuring the good parallel light and measuring it, combined with the long/short wavelength measurement results, a large distance and high precision measurement can be obtained.

For other purposes and functions of the present invention, please refer to the drawings and the embodiments, which are described in detail below.

11‧‧‧Fixed mirror

12‧‧‧Mobile mirror

2‧‧‧ Length measuring device

21‧‧‧First laser unit

22‧‧‧Second laser unit

23‧‧‧First temperature controller

24‧‧‧Second temperature controller

25‧‧‧Laser controller

26‧‧‧First Polarizing Beam Splitter

27‧‧‧ Beam isolator

28‧‧‧Fiber collimator

29‧‧‧ Beam expander

31‧‧‧ Splitter

32‧‧‧wavelength detection unit

33‧‧‧Fixed mirror

330‧‧‧Resonance cavity

34‧‧‧Mobile mirror

35‧‧‧Mobile unit

36‧‧‧Secondary polar splitter

37‧‧‧First line array detector

38‧‧‧Second line array detector

39‧‧‧First Signal Processing Unit

41‧‧‧Second signal processing unit

42‧‧‧ processor

43‧‧‧Display and output unit

Figure 1 is a schematic diagram of the architecture of a known Fabry-Perot interferometer.

Figure 2 is a schematic illustration of the location of the initial interference fringes recorded during the Fabry-Perot interference fringe experiment.

Figure 3 is a schematic diagram showing the position of the interference fringes after the displacement of the moving mirror in the Fabry-Perot interference fringe experiment.

4 is a schematic view of a length measuring device of the present invention.

FIG. 5 is a schematic diagram of obtaining a measurement distance corresponding to a displacement distance by performing a semiconductor laser with a wavelength of 657.2 nm.

FIG. 6 is a schematic diagram of the measurement distance corresponding to the displacement distance of the semiconductor laser of the invention with a wavelength of 658.1 nm.

FIG. 7 is a schematic diagram showing the displacement distance corresponding to the measurement distance by performing absolute distance measurement at a dual wavelength composed of wavelengths of 657.2 nm and 658.1 nm.

The invention adopts a dual-wavelength synthetic Fabry-Perot interferometer combined with the architecture of two line array detectors, uses a temperature control device to ensure the stability of the laser wavelength, and expands the laser beam by a collimator. After measuring the good parallel light, the measurement is performed at the same time, and the measurement of the two wavelengths is combined, and the measurement results of the two wavelengths are combined to obtain a large-distance and high-precision measurement.

As shown in FIG. 4, the length measuring device 2 of the present invention comprises a first laser unit 21, a second laser unit 22, a first temperature controller 23, a second temperature controller 24, a laser controller 25, First polarized beam splitter 26, beam isolator 27, fiber collimator 28, expansion Beam mirror 29, beam splitter 31, wavelength detecting unit 32, fixed mirror 33, moving mirror 34, moving unit 35, second polarizing beam splitter 36, first line array detector 37, second line array detector 38 The first signal processing unit 39, the second signal processing unit 41, the processor 42, and the display and output unit 43 are composed.

The first laser unit 21 and the second laser unit 22 are line-polar laser units; the laser controller 25 electrically connects the first laser unit 21 and the second laser unit 22 to control the first laser. The unit 21 and the second laser unit 22 output a laser beam. The first temperature controller 23 and the second temperature controller 24 respectively control the temperatures of the first laser unit 21 and the second laser unit 22, so that the first laser unit 21 and the second laser unit 22 output a stable wavelength. The first signal processing unit 39 is electrically coupled to the first line array detector 37; the second signal processing unit 41 is electrically coupled to the second line array detector 38. The processor 42 is electrically connected to the laser driver 25, the wavelength detecting unit 32, the first signal processing unit 39, the second signal processing unit 41, and the display and output unit 43, respectively. The moving unit 35 is combined with the moving mirror 34 to drive the moving mirror 34 to adjust the distance between the fixed mirror 33 and the moving mirror 34 corresponding to the length to be measured. The first line array detector 37 and the second line array detector 38 may be linear CCD, CIS, LDA, CMOS, etc., respectively.

When the length measuring device 2 of the present invention performs measurement, first adjust the polarization directions of the first laser unit 21 and the second laser unit 22 so as to be orthogonal to each other (vertical); if the first laser unit 21 is biased The direction is a vertical polarized laser beam with a wavelength of λ ₁ , and the second laser unit 22 is adjusted to a horizontally polarized laser beam having a wavelength of λ ₂ and matched with the first polarized beam splitter 26 to The vertical polarized laser beam of the first laser unit 21 is completely penetrated, and the horizontal polarized laser beam of the second laser unit 22 is completely reflected, and then the two orthogonal polarized laser beams are coincident, and sequentially Passing through the beam isolator 27, the fiber collimator 28, and the beam expander 29 (the present invention also eliminates the need for the beam isolator 27 and the fiber collimator 28, allowing the laser beam to pass directly through the beam expander 29) to a split beam The prism 31 is divided into two laser beams, each of which contains two orthogonal polarized laser beams.

The reflected laser beam enters the wavelength detecting unit 32 to monitor the wavelength of the laser beam, and the other laser beam enters the resonant cavity 330 formed by the fixed mirror 33 and the moving mirror 34 to form a multi-interference laser beam, and then Re-entering the second polarized beam splitter 36 at the other end; the second polarized beam splitter 36 separates the vertical polarized laser beam from the horizontal polarized laser beam, wherein the vertical polarized laser beam Passing through the second polarizing beam splitter 36, being received by the first line array detector 37, and performing pre-processing by the first signal processing unit 39; and the horizontally polarized beam is split by the second polarizing beam The 稜鏡36 reflects, is received by the second line array detector 38, and is subjected to pre-processing by the second signal processing unit 41; the processor 42 processes the signals transmitted by the first signal processing unit 39 and the second signal processing unit 41. Then, the display and output unit 43 is controlled to output the measurement image and the measurement result.

5 and FIG. 6 respectively, an experiment is performed on a semiconductor laser (first laser unit 21, second laser unit 22) having two wavelengths λ ₁ and λ ₂ of 657.2 nm and 658.1 nm, respectively, each time moving The mirror 34 was unidirectionally displaced by 100 nm (nano) with a total displacement of 16 times. In Figs. 5 and 6, the horizontal axis represents the position (displacement distance) of the moving mirror 34, and the vertical axis represents the measured distance in units of nm. Since the position of the moving mirror 34 is controlled by a precise motor platform (moving unit 35) during the experiment, a specific distance of 100 nm is moved each step by step. An optical scale having a resolution of 4 nm is mounted on the moving shaft of the motor, so that the reading distance before and after the movement is recorded, and the distance moved by the moving mirror 34 can be determined. 7 is a result of absolute distance measurement of dual wavelengths composed of wavelengths λ ₁ and λ ₂ of 657.2 nm and 658.1 nm, respectively. In FIG. 7, the horizontal axis represents the position (displacement distance) of the moving mirror 34, and the vertical axis is The measured distance is in μm (micrometers). The experiment was carried out in steps, moving 21 microns each time, moving a total of 20 steps totaling 420 microns. From the above experimental results, the single-wavelength distance measurement and the dual-wavelength absolute distance measurement are respectively proportional to the actual displacement distance, so that the length to be measured can be obtained from the measurement distance, that is, the absolute distance measurement value of the dual wavelength plus one of them. The distance measurement of the single wavelength is equal to the length value to be measured. For example, the absolute distance measurement value of the dual wavelength is 70 μm, and if the distance measurement value of a single wavelength is 200 nm, the length value to be measured is 70 μm plus 200 nm is equal to 70.2 μm.

Since semiconductor lasers are very sensitive to temperature, generally have a sensitivity of about 0.2, so how to reduce the change of wavelength is the key to the correctness of length measurement. Therefore, the present invention adopts the temperature control mode of the PID to control the temperature at a certain temperature, and the temperature change amount is within 0.01 ° C, so that the wavelength can be maintained constant.

Since any optical component has a surface reflection phenomenon, if the reflected light returns to the inside of the laser, the laser will be unstable. In addition to causing a change in output power, the wavelength will also drift, so the reflected spurs must be isolated. The beam, therefore, the beam isolator 27 of the present invention is used to block stray light; if there is no beam isolator 27, the length measuring device of the present invention can operate, but with a slight reduction in accuracy.

In addition, different laser powers have different laser wavelengths, and when the laser power is too high, linear CCD, CIS, LDA, CMOS (charge coupled components, contact photoelectric sensing elements, semiconductor photodetector arrays, Photodetectors such as complementary MOS are saturated, while insufficient power reduces the signal-to-noise ratio of linear CCD, CIS, LDA, CMOS, etc., both of which reduce measurement accuracy. To solve this problem, it must be ensured that the laser power is not too high. Generally, the attenuation power is used to reduce the power of the laser. When the wavelength is changed, the output power of the laser may be reduced. The shutter time (ie, integration time) of photodetectors such as CCD, CIS, LDA, CMOS can improve the signal strength to solve this problem.

The resonant cavity 330 used for measuring the length of the present invention is a planar resonant cavity, which is composed of two planes of high reflectivity plane mirrors (fixed mirror 33, moving mirror 34), so that it has good sharpness and fineness, and the stripe energy is very concentrated. Clear and easy to distinguish. As long as the relative angle between the mirror surface of the fixed mirror 33 and the mirror surface of the moving mirror 34 is adjusted before use, a small number of interference fringes are called, and then the approximate position of the stripe is discriminated by the signal, and then further The window method is used to define the range of processing, and then the squared centroid method can determine the exact position of the stripe. After each time the image is captured, the position of the stripe is judged and the difference from the previous position is analyzed. Since the pitch of the stripe is constant and corresponds to the length of the half wavelength, the distance of the movement can be derived from the change of the position to obtain the distance. At the resolution of the meter level, when the positional variation of the two wavelengths is obtained separately, the change information of the large distance (absolute distance) can be obtained according to the method of wavelength synthesis, and is not limited by the half wavelength of λ ₁ and λ ₂ . Combined with the long/short wavelength measurement results, large distance and high precision results can be obtained.

If the multi-color spectroscopic ray is combined with the coating technology, the invention can combine more wavelengths of laser light and synthesize more synthetic wavelengths, so that the interferometer has a larger use range.

The invention utilizes the measurement architecture of the Fabry-Perot interferometer, uses a temperature controller to ensure the stability of the laser wavelength, expands the laser beam by the collimator, and becomes a good parallel light, and then performs the measurement operation, combining The measurement results of the long/short wavelengths λ ₁ and λ ₂ can be measured with large distance and high precision.

The architecture of the present invention can also receive and detect interference patterns formed by three or four wavelengths at the same time, so that interference results of at least two wavelengths can be simultaneously measured, except The speed of the measurement is increased, and the influence of the differential mode error is reduced, and the accuracy of the detection is improved.

The above descriptions are only examples of the use of the technical content of the present invention, and any modifications and variations made by those skilled in the art using the present invention are within the scope of the patent claimed.

2‧‧‧ Length measuring device

21‧‧‧First laser unit

22‧‧‧Second laser unit

23‧‧‧First temperature controller

24‧‧‧Second temperature controller

25‧‧‧Laser controller

26‧‧‧First Polarizing Beam Splitter

27‧‧‧ Beam isolator

28‧‧‧Fiber collimator

29‧‧‧ Beam expander

31‧‧‧ Splitter

32‧‧‧wavelength detection unit

33‧‧‧Fixed mirror

330‧‧‧Resonance cavity

34‧‧‧Mobile mirror

35‧‧‧Mobile unit

36‧‧‧Secondary polar splitter

37‧‧‧First line array detector

38‧‧‧Second line array detector

39‧‧‧First Signal Processing Unit

41‧‧‧Second signal processing unit

42‧‧‧ processor

43‧‧‧Display and output unit

Claims (8)

A length measuring device comprises: a first laser unit, which is a line-polar laser unit; a second laser unit, which is a line-polar laser unit; and a laser-driven controller electrically connected to the first a laser unit and the second laser unit for controlling the first laser unit and the second laser unit to output a laser beam; a first polarized beam splitter; a beam expander; a fixed mirror; Moving the mirror, nearly parallel to the fixed mirror; forming a resonant cavity between the moving mirror and the fixed mirror; a second polarized beam splitter; a first line array detector; a second line array detector; a signal processing unit electrically connecting the first line array detector; a second signal processing unit electrically connecting the second line array detector; a display and output unit; and a processor electrically connecting the laser drive The first signal processing unit, the second signal processing unit, and the display and output unit; wherein the polarization directions of the first laser unit and the second laser unit are orthogonal to each other; The polarization direction of the firing element is a vertical polarized laser Beam wavelength λ 1, the polarization direction of the second laser unit is a horizontal polarized laser beam having a wavelength of λ 2, and cooperates with the first polarizing beam splitter such that the vertical polarized laser beam of the first laser unit penetrates the first polarized beam splitter, and the horizontal offset of the second laser unit The polar laser beam is reflected by the first polarized beam splitter, and then the two orthogonal polarized laser beams are coincident, and after passing through the beam expander, the resonance formed by the fixed mirror and the moving mirror is entered. a plurality of interfering laser beams are formed in the cavity, and then enter the second polarized beam splitter; the second polarized beam splitter separates the vertical polarized laser beam from the horizontal polarized laser beam in the laser beam a vertical polarized laser beam passes through the second polarized beam splitter, is received by the first line array detector, and is subjected to pre-processing by the first signal processing unit, and the horizontally polarized laser beam And being reflected by the second polarized beam splitter, received by the second line array detector, and being subjected to pre-processing by the second signal processing unit; the processor processing the first signal processing unit and the first The signal transmitted by the two signal processing units is composed of the λ 1 and λ 2 double waves The absolute distance measurement value plus λ 1 and λ 2 of the dual wavelength measurement distance of one single wavelength, a length value is obtained to be measured, and controlling the display and output unit outputs the measurement results. The length measuring device according to claim 1, further comprising a splitter and a wavelength detecting unit; the wavelength detecting unit is electrically connected to the processor; and after the laser beam passes through the beam expander, first enters The split pupil is divided into two laser beams, each of which contains two orthogonal polarized laser beams, wherein the reflected laser beam enters the wavelength detecting unit to monitor the wavelength of the laser beam. Another laser beam enters the fixed mirror. The length measuring device of claim 2, wherein the first line array detector and the second line array detector are linear charge coupled elements, contact photoelectric sensing elements, and semiconductor photodetectors, respectively. One of an array and a complementary MOS photodetector. The length measuring device of claim 2, wherein the beam splitter is a multi-color splitter. The length measuring device of claim 3, wherein the splitter is a multi-color splitter. The length measuring device according to any one of claims 1 to 5, further comprising a first temperature controller and a second temperature controller; the first temperature controller and the second temperature controller The temperatures of the first laser unit and the second laser unit are respectively controlled such that the first laser unit and the second laser unit output a stable wavelength. The length measuring device of claim 6, further comprising a beam isolator; the two orthogonal polarized laser beams are coincident, and the beam isolator is passed through the beam expander before passing through the beam expander Block stray light from reflection. The length measuring device of claim 7, further comprising a fiber collimator; the laser beam passing through the beam isolator first passes through the fiber collimator and then passes through the beam expander mirror.