KR101746693B1 - absolute distance measuring apparatus using multi-two-color interferometer - Google Patents
absolute distance measuring apparatus using multi-two-color interferometer Download PDFInfo
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- KR101746693B1 KR101746693B1 KR1020150179328A KR20150179328A KR101746693B1 KR 101746693 B1 KR101746693 B1 KR 101746693B1 KR 1020150179328 A KR1020150179328 A KR 1020150179328A KR 20150179328 A KR20150179328 A KR 20150179328A KR 101746693 B1 KR101746693 B1 KR 101746693B1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B9/00—Measuring instruments characterised by the use of optical techniques
- G01B9/02—Interferometers
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B9/00—Measuring instruments characterised by the use of optical techniques
- G01B9/02—Interferometers
- G01B9/02001—Interferometers characterised by controlling or generating intrinsic radiation properties
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B9/00—Measuring instruments characterised by the use of optical techniques
- G01B9/02—Interferometers
- G01B9/02001—Interferometers characterised by controlling or generating intrinsic radiation properties
- G01B9/02007—Two or more frequencies or sources used for interferometric measurement
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- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
- G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
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- G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
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Abstract
It is an object of the present invention to provide a method and an apparatus for measuring geometric length by economically and efficiently measuring lengths using an interferometer using laser light of two wavelengths and maximizing accuracy by compensating for errors according to a refractive index of a medium, And an absolute length measuring device using the multiple double wavelength laser interferometer which can measure the absolute length by using multiple such interferometers having wavelengths.
Description
The present invention relates to an absolute length measuring apparatus using a multiple double wavelength laser interferometer and, more particularly, to an absolute length measuring apparatus using an absolute length measuring apparatus which is capable of measuring an absolute length (length) economically and efficiently while maximizing the accuracy, And more particularly, to an apparatus for measuring an absolute length using a double-wavelength laser interferometer.
Measurement of the length in various technical fields is very basic. In the fields of geography measurement, aeronautical satellites, etc., the length is measured in km units. In general industrial fields such as production and construction, The range of lengths to be measured varies depending on the field. Depending on the length range to be measured, the accuracy required in the field may vary accordingly. On the other hand, mechanical and electronic technologies have recently been combined in various fields, such as the application of semiconductor technology to manufacture nanomachines and the application of various electronic controls to realize ultra-precise processing. In such a field, the accuracy required for length measurement is required to be as high as about μm or nm, compared with other fields.
Various studies have been made to realize length measurement performed with such an extremely high accuracy. Japanese Patent Laid-Open Publication No. 2009-288159 ("Distance Measuring Apparatus and Optical Interferometer with this, Optical Microscope", 2009.12.10, precedent) Researches have been actively conducted to increase the accuracy of length measurement using a laser or the like by applying optical technology. The basic principle of such a laser interferometer is to separate the light irradiated from the light source into the reference light and the measurement light and to compare the measurement light returned from the measurement object with the reference light to calculate the light path difference. Can be measured with high precision.
The length measurement technique using the laser interferometer can be classified into various kinds, and it can be classified into the kind of the light source first. When a pulsed laser is used as the light source of a laser interferometer, the length can be measured at the level of nm precision, but the measurable range is limited to an integer multiple of the pulse interval. Or a continuous wave laser (CW laser) is used, it is also possible to measure the length to the nm precision level, but it has a disadvantage that the measurable area itself is narrow.
In addition, it can be classified into relative length measurement and absolute length measurement depending on whether the distance is measured by a laser interferometer or whether the position of the measurement object is changed or fixed. In the relative length measurement method, the relative displacement between the initial position and the final position of the measurement surface is measured by continuously integrating the phase change of the interference fringe generated by the movement of the measurement plane while changing the distance between the laser interferometer and the measurement target , And a method of obtaining a desired length measurement value as an accumulated value of these relative displacements, which is currently in commercial use and widely used. This method has the merit that calculation is simple and quick measurement is possible. However, there is an error in the process of moving the measurement plane as desired, and since the phase change is continuously integrated, various error components accumulated in the signal are accumulated There is also.
The absolute length measurement method is based on a study of a system capable of measuring the absolute length while applying the principle of a conventional interferometer to solve this problem. As a part of such research, the same distance is measured using two or more wavelengths There is a multi-wavelength interferometer. Although the multi-wavelength interferometer has the advantage of being able to measure the absolute length with high measurement uncertainty, its use is limited to a specific field where the initial prediction of the length is possible due to the disadvantage that the length to be measured can be estimated within a certain range. There is also a problem that it is difficult to obtain a high level of measurement uncertainty as much as the relative displacement measurement due to light source stability problems and algorithm problems.
On the other hand, the error factor which should be considered important in both the relative displacement measurement method and the absolute displacement measurement method is the influence of the environmental variable. Ideally, it is possible to calculate the exact length by using the difference in optical path between the measurement light and the reference light when measuring the length using the laser. However, in reality, the refractive index of the medium (air in most cases) Should be considered.
In the past, the refractive index of air was calculated from the distance measured in vacuum and the distance measured in air by using a laser interferometer. However, this method is not used at present because of the limitation of measuring a certain distance. At present, there are widely used environmental sensors including environmental sensors for measuring environmental variables such as temperature, pressure, and humidity, which affect the refractive index of air, and correcting the measured distances using environmental variable values. However, the environmental sensor can only measure the environmental variable at a specific point in the space where the laser light travels as shown in the figure. If the air in the space is completely homogeneous, there is no problem in using only the environmental information of the measurement point. However, since it is difficult to actually assure that the air is completely uniform in the entire space, The environmental information in the space is different, so that it may result in inaccurate results when calculating the length. In order to overcome this problem, a simple solution is to distribute a plurality of environmental sensors in the space and measure the distribution of the environmental information, and to reflect this in the calculation formula of length. However, There is a problem that the cost for device configuration increases while accuracy improves, which is very uneconomical. In addition, the fundamental problem in the case of using the environmental sensor is that it is difficult to overcome the limitation of the point measurement of the environmental sensor, and it is difficult to apply it in the atmospheric environment where the environmental change is severe, such as the outdoor environment.
As described above, a measurement apparatus capable of compensating an error according to a medium refractive index depending on an environmental variable and measuring an absolute length with high precision has been steadily studied and developed.
SUMMARY OF THE INVENTION The present invention has been made in order to solve the problems of the conventional art as described above, and it is an object of the present invention to provide an apparatus and a method for measuring a geometric length by using an interferometer using laser light of two wavelengths, And the absolute length measurement using a multiple wavelength laser interferometer which can measure the absolute length by using multiple such interferometers having different wavelengths, Device.
Absolute length measuring apparatus using a multiple two-wavelength laser interferometer according to the present invention for achieving the object as described above, one another a plurality of elementary light having different wavelengths (λ 1, λ 2, λ 3, λ 4) and the base a light source for forming a plurality of laser consisting of at least one second comparison blend Fine light (λ SHG) is selected such that light of the light irradiated toward the object to be measured along the same optical path; A splitting unit disposed on an optical path between the light source and the object to divide the light by reflecting part of the light and passing the remaining part; The light reflected from the divided portion is referred to as a reference light, and the light that passes through the divided portion, reflected by the measured object, and reflected by the divided portion sequentially passes through the optical path of the reference light A first dichroic mirror for separating the reference-based light and the reference-reference light by reflecting or passing light along the wavelength; A second dichroic mirror disposed on the optical path of the measurement light for separating the measurement-based light and the measurement-comparison light by reflecting or passing the light according to the wavelength; A light measuring unit for measuring reference-based light, reference-comparing light, measurement-based light, and measurement-comparison light reflected or passed through the first dichroic mirror and the second dichroic mirror; A calculation unit for calculating a geometric absolute length to the object by comparing the reference-base light, the reference-comparison light, the measurement-basis light, and the measurement-comparison light measured by the light measuring unit; . ≪ / RTI >
In this case, the absolute length measuring apparatus uses a plurality of base lights (? 1 ,? 2 ,? 3 ,? 4 ) and a comparison light (? SHG ) (L) < / RTI >
(From here,
L: geometrical length to the object to be measured,
D i is the optical path length measured by the i-th basic light wavelength (? I ) (the fundamental light which is the source of the comparison light in which the i-th basic light is the secondary harmonic light)
D SHG is the optical path length measured by the comparative light wavelength? SHG ,
A: amplification factor,
The absolute length measuring apparatus may further comprise an absolute length measuring unit that uses a plurality of base lights (λ 1 , λ 2 , λ 3 , λ 4 ) and defines the range of the solution L c as an initial estimate of a predetermined geometric absolute length The geometric absolute length can be calculated by applying the summation method according to the equation.
(From here,
i: index of the fundamental light,
N: the total number of the base lights,
m i : i < th > fundamental light phase integral part,
e i : i th basic optical phase fractional part,
L c : the year determined by the concatenation,
d: phase tolerance)
Or the absolute length measuring device is configured to use the plurality of base lights (? 1 ,? 2 ,? 3 ,? 4 ) and the comparison light (? SHG ) The compensated geometric length (Li) is calculated,
(From here,
L i is the geometrical length to the measured object measured by the i-th basis light,
D i is the optical path length measured by the i-th basic light wavelength (? I ) (the fundamental light which is the source of the comparison light in which the i-th basic light is the secondary harmonic light)
D SHG is the optical path length measured by the comparative light wavelength? SHG ,
A: amplification factor,
Using a geometric length (Li) compensated for the medium refractive index to the measured object by the i-th basis light, and defining a range of the solution (L c ) as an initial estimate of the predetermined geometric absolute length To calculate the geometric absolute length measured by the i < th > base light,
(From here,
i: index of the fundamental light,
N: the total number of the base lights,
m i : i < th > fundamental light phase integral part,
e i : i th basic optical phase fractional part,
L c : the year determined by the concatenation,
d: phase tolerance)
The geometric absolute length Li compensated for the medium refractive indexes obtained for each of the N th i-th basis light can be averaged by the following equation to calculate the geometric absolute length L compensated for the medium refractive index.
The light source unit may include an optical comb and an external laser to generate a laser beam having a plurality of reference frequencies spaced apart from each other at a predetermined interval in a frequency domain and to use the frequency of the laser beam generated from the optical comb So that the frequency of the laser beam generated from the external laser is stabilized at a predetermined frequency.
At this time, the light source unit further includes an atomic clock connected to the optical comb and a phase locked loop (PLL) connected to the atomic clock, and the external laser is connected to the phase lock circuit The laser light generated in the light comb and the laser light generated in the external laser are synchronized so that the frequency of the laser light generated in the external laser is stabilized. More specifically, the light source unit may be formed such that the frequency of laser light generated in the external laser is stabilized according to the following equation.
f i = if r + f o
f DFB = if r + f o + f b
(From here,
f DFB is a frequency of laser light generated in the external laser,
f i is a stabilization reference frequency selected for stabilizing the external laser light frequency among reference frequencies of the laser light generated in the light comb,
f r is a repetition rate of laser light generated in the light comb,
if r : the i-th repetition rate (i is a natural number) closest to the stabilization reference frequency among the repetition rate values smaller than the stabilization reference frequency,
f o is an offset frequency between the stabilization reference frequency and the i th repetition rate,
f b is a bit frequency between the reference frequency for stabilization and the frequency of laser light generated by the external laser)
The light source unit may include an atomic clock connected to the optical comb, a Fabry-Perot filter connected to the optical comb, and a fiber Bragg grating (FBG) connected to the Fabry-Perot filter. Wherein the laser light generated in the optical comb passes through the filter unit and only a part of the mode is selected and the laser light selected in the partial mode is incident on the external laser by the circulator, So that the frequency of the laser beam generated from the laser beam can be stabilized. More specifically, the light source unit may be formed such that the frequency of laser light generated in the external laser is stabilized according to the following equation.
f i = if r + f o
f DFB = if r + f o
(From here,
f DFB is a frequency of laser light generated in the external laser,
f i is a stabilization reference frequency selected for stabilizing the external laser light frequency among reference frequencies of the laser light generated in the light comb,
f r is a repetition rate of laser light generated in the light comb,
if r : the i-th repetition rate (i is a natural number) closest to the stabilization reference frequency among the repetition rate values smaller than the stabilization reference frequency,
f o is an offset frequency between the stabilization reference frequency and the i th repetition rate,
Further, the light source unit may include an optical coupler (OC) that divides and advances the laser light, a periodically poled lithium niobate (PPLN) that folds the wavelength of light passing through by generating a second harmonic wave of the incident light, (DM, Dichroic Mirror) through which the light passes or reflects, and one of the lights split by the optical coupler travels toward the dichroic mirror to pass through the dichroic mirror to form a base light , And the other of the light split by the optical coupler passes through the PPLN and then travels toward the dichroic mirror to form a comparison light that is a second harmonic of the base light and is reflected by the dichroic mirror, The light is made to travel to the same optical path so that the light emitted from the light source portion has a basic light wavelength and a comparative light wavelength.
According to the present invention, in measuring the geometric absolute length, an absolute length measurement method of measuring one distance with laser light having a plurality of different wavelengths is used instead of the relative distance measurement method in which the measurement is performed while varying the position of the measurement object The measurement accuracy can be improved because the measurement error can be determined by one measurement and no cumulative error is generated.
In addition, according to the present invention, as described above, a different wavelength that is relatively different from the wavelengths used for distance measurement is further used, and a medium (air) that varies depending on environmental variables such as temperature, pressure, It is possible to maximize the accuracy of the measurement of the geometrical absolute length.
In the present invention, since measurement is performed using a plurality of wavelengths, it is necessary to stably generate laser light having a precise wavelength as desired. In the present invention, various wavelengths as desired in a laser light source unit Thereby minimizing the error amplification problem that may occur when measuring the geometrical length with a laser having a plurality of wavelengths, thereby further improving the accuracy of the final calculated geometrical length. That is, according to the present invention, it is possible to maximize the improvement of the accuracy of the calculation of the geometric distance by eliminating the error and the cause of the amplification.
In addition, since the present invention does not require expensive additional parts in terms of the length calculation principle, it is advantageous in economical efficiency since the cost of manufacturing the apparatus is not so high while the accuracy of the measuring apparatus is greatly improved have. Of course, there is also an advantage in that the apparatus of the present invention is advantageously applied to an actual industrial field based on such economical efficiency.
1 shows a principle of length measurement using a two-wavelength laser interferometer.
2 shows the principle of absolute length measurement using a multi-laser interferometer.
3 is an embodiment of an absolute length measuring apparatus using a multiple double-wavelength laser interferometer according to the present invention.
FIG. 4 shows another embodiment of the absolute length measuring apparatus using the multiple double-wavelength laser interferometer of the present invention.
5 is an embodiment of a stabilized wavelength generating light source.
6 is another embodiment of a stabilized wavelength generating light source.
7 is an embodiment of the stabilization wavelength generation principle.
8 is another embodiment of the stabilization wavelength generation principle.
9 is an embodiment of the basic light-comparison light generating light source.
Hereinafter, an absolute length measuring apparatus using the multiple double-wavelength laser interferometer according to the present invention will be described in detail with reference to the accompanying drawings.
The principle of the absolute length measuring apparatus using the multiple double wavelength laser interferometer of the present invention is briefly described as follows. The principle of the double wavelength laser interferometer that obtains the compensated geometric length of the medium refractive index and the length of one length By combining the principle of a multi-laser interferometer that measures the absolute length at one time, the medium refractive index is compensated to measure the geometric absolute length. That is, by measuring the absolute length using a multi-laser interferometer configuration, by introducing a dual-wavelength laser interferometer configuration in the process of measuring the absolute length, the measured absolute length is the compensated geometric absolute length of the medium refractive index.
Therefore, in order to understand the apparatus of the present invention, it is necessary to first understand the principle of obtaining the geometrical length compensated by the medium refractive index using a two-wavelength laser interferometer and the principle of measuring the absolute length using a plurality of wavelengths using a multi- shall. Each principle will be described below.
[1] Principle of length measurement using two-wavelength laser interferometer
FIG. 1 is a view for explaining the principle of length measurement using a two-wavelength laser interferometer, which discloses a simple two-wavelength laser interferometer consisting of only a minimum number of components. 1, the basic structure of a two-wavelength laser interferometer includes a
The
On the other hand, in the example of Fig. 1,
In this way, in the example of FIG. 1, an optical path is formed so that a separate optical measuring unit can separately receive the reference light and the measuring light, respectively, and the reference light and the measuring light are separately separated into the base light and the comparison light, The reference light, the reference light, the reference light, the measurement light, and the measurement light can be separately measured. However, because the wavelengths of the beams are different, it is not necessary to configure the optical measuring units separately so that various types of beams can be incident on the same optical measuring unit through the same optical path and can be separated. For example, in the example of FIG. 1, only the dichroic mirrors are removed, so that the basic light and the comparison light can be incident at a time In addition, in the example of FIG. 1, the reference light and the measurement light can be incident at once, as long as the reference light is reflected by the mirror once. That is, the optical system may be modified in various ways by appropriately adding or removing mirrors, other optical splitters, and other dichroic mirrors that can change the optical path in the example of FIG.
The principle of length measurement using a two-wavelength laser interferometer is now described.
In a typical laser interferometer, one laser beam is divided into two, one is used as reference light and the other is used as measurement light that is reflected from the object to be measured and fired to the object to be measured. By using the phase difference between the reference light and the measurement light, The optical path length difference between the light beams is calculated and the distance (i.e., length) to the measurement object is measured. In this case, as described briefly above, there is no problem in the case where there is no medium in which light travels (in most cases, air), that is, in a vacuum state. However, in the presence of a medium, (OPL, Optical Pass Length, D) measured using a laser interferometer.
The relationship between the optical path length (OPL) D and the geometric length (length to be actually measured) L is as follows.
[Equation 1]
&Quot; (2) "
In Equation (1), n is the refractive index of the medium (generally air) filled in the space where the laser beam travels, and? Represents the wavelength of the laser used for the measurement. As described above, by using the phase? Obtained by dividing the light irradiated from the light source part into the measurement light and reference light by the splitting part, then proceeding to the object to be measured and then comparing the measurement light returned with the reference light to the reference light, Can be calculated. The relationship between the phase? And the geometrical length L is shown in Equation (2). (Lambda in Equation 1 indicates a general 'wavelength' and lambda 0 in Equation 2 means a wavelength of a laser used for measurement in the apparatus shown in FIG. 1). That is, Equations 1 and 2 It is possible to calculate the length to be actually measured, that is, the geometric length L value.
&Quot; (3) "
However, as described above, the refractive index n is a function that depends on environmental variables such as pressure, temperature, position, humidity, etc. in addition to the laser wavelength as shown in Equation (3). Accordingly, the error generated in the calculation of the refractive index n is accumulated in the calculation of the geometric length L as it is, which is a cause of lowering the accuracy.
In the two-wavelength laser interferometer, in order to eliminate the refractive index error as described above, the same distance is measured using laser light having two different wavelengths, and the geometric length L is calculated through the following equation.
&Quot; (4) "
&Quot; (5) "
In the equations (4) and (5), D represents the optical path length measured by the base light, D SHG represents the optical path length measured by the comparison light ,? Represents the base light wavelength and? SHG represents the comparison light wavelength The comparison light is not necessarily the second harmonic wave of the fundamental light. In this case, the comparison light is not necessarily the second harmonic wave of the fundamental light. In other words, variable associated with is also expressed by λ 1, D 1, etc., a variable relating to the comparative light is λ 2, D 2, and so on). Also, A represents the amplification factor. In the case of a dry air environment (ie, when the humidity value is close to 0), the A value is a constant value of 141.41 when the wavelength is 1555 nm or 777.5 nm. The dot is well known. If a light source of different wavelength is used, the amplification value may be different from 141.41, but the amplification value is a constant value anyway.
That is, in the case of the geometrical distance measuring method using the conventional laser interferometer, since the geometric distance was calculated using Equations 1 and 2, the refractive index of the medium (air) was necessarily required. However, When measuring the geometric distance using a laser interferometer, it is not necessary to measure the refractive index if it is a dry air environment, and it is possible to use a known constant A value. In other words, according to the equation (4), the optical path length D measured by the base light and the optical path length D SHG measured by the comparison light irrespective of the refractive index n (λ) and n (λ SHG ) L can be calculated.
Therefore, a length measuring apparatus using a two-wavelength laser interferometer does not need to have an environmental sensor unlike a conventional length measuring apparatus using a laser interferometer. In addition, since the environmental variable on the actual light path is not measured but the environmental variable is measured at a separate measurement point in the conventional measurement of the environmental variable, when calculating the refractive index used in calculating the geometric length when the medium is uneven, However, according to the present invention, since the refractive index is not used in calculating the geometric length, the problem of generating errors due to such environmental influences can be essentially eliminated.
[2] Absolute length measurement principle using multiple laser interferometers
Fig. 2 is a view for explaining the principle of absolute length measurement using a multi-laser interferometer, which discloses a simple multi-laser interferometer consisting of only a minimum number of components. 2, the basic structure of a multiple laser interferometer includes a
The
The principle of length measurement using a multi-laser interferometer is now described.
Assuming that there is no influence due to the refractive index of the medium (in the case of vacuum), or that the error due to the refractive index of the medium is compensated by using the principle described in the paragraph [1] , the optical path length D is equal to the geometrical length L. In the following, it is assumed that the optical path length is measured in an environment where the error due to the medium refractive index is compensated or the medium refractive index has no influence, that is, the optical path length D is equal to the geometric length L.
As described briefly in the measurement principle of the conventional laser interferometer, the optical path length difference calculated from the phase difference between the measurement light and the reference light is a geometric length. In this case, the following problem arises. For example, when the phase difference between the measurement light and the reference light is measured to be 180 degrees, the difference between the optical path lengths of the measurement light and the reference light is 1/2 wavelength length, and 1 1/2 wavelength length, 2 + 1/2 wavelength length ... Can not actually be specified. That is, the optical path length obtained by the phase difference between the measurement light and the reference light remains as an unknown number of lengths corresponding to an integral multiple of the wavelength. Taking this into consideration, the relationship between the optical path length, that is, the geometrical length L, and the wavelength? Can be expressed as follows.
&Quot; (6) "
In Equation (6), L represents the optical path length or geometrical length,? Represents the wavelength of the laser beam used for measurement, m represents the integer part of the measured phase, and e represents the fractional part of the measured phase. Here, lambda is a known value, e is a value that can be determined by measurement, but m is an unknown number, so that it is not possible to calculate the geometric absolute length by itself.
However, in a multi-laser interferometer, the same distance is simultaneously measured with a plurality of laser beams having different predetermined wavelengths with different initial wavelengths of the predetermined geometrical absolute lengths, and an exactfraction method is used based on the initial estimates, The length can be calculated.
The initial estimates of the geometric lengths to be measured in a multi-laser interferometer can be predetermined in a variety of ways. For example, an error estimate may be used as an initial estimate of the length measurement using a general laser interferometer principle. Alternatively, a method of obtaining an initial estimate using a wavelength sweep laser may be used. Alternatively, the initial estimate may be determined from an entirely different perspective, such as: In most cases, the length of the object to be measured is not completely known. For example, if you have a product with a design length of 10 cm, you may want to measure the length of the product in μm or nm. In such a case, the length of the design standard that is already known may be used as the initial estimate without measuring the length in any other way. As will be described in more detail below, the multi-laser interferometer is intended to measure the exact length up to the μm or nm level, so the initial estimate may be obtained in a less accurate manner. That is, in addition to the above-described example, the initial estimate value can be determined in advance by various methods.
Next, when the same distance is measured simultaneously by a plurality of laser beams having different wavelengths, the following relationship is established.
&Quot; (7) "
In Equation (7), i represents an index for distinguishing a plurality of laser beams, and N represents the total number of the plurality of laser beams. m i represents the phase integral measured by the i-th laser beam, and e i represents the phase decimator measured by the i-th laser beam. In Equation (7), since the number of equations is N and the number of unknowns is N + 1 (N of integer part + 1 value of length), a unique solution can not be obtained for all areas. However, Can be estimated within a sufficiently small range, the absolute length can be obtained analytically through the exactfraction method.
&Quot; (8) "
In Equation 8, E (X) denotes a function that takes a fractional part of the X, λ i is the i-th denotes a wavelength of laser light, i e represents a fractional part of the phase measured by the i-th laser beam. Also, L c is the initial estimate value of the geometric absolute length (predetermined by being measured or known in various manners as previously described), and d is the predetermined phase tolerance value. Similar to the earlier determination of the initial estimate, the phase tolerance d value can be determined appropriately by the accuracy level of the geometric absolute length to be sought. For example, in the case of using the wavelength sweep laser among the above-mentioned examples of determination of the initial estimation value, the uncertainty that can be calculated by analyzing the length measurement using the wavelength sweep laser and used as the d value can be utilized. Alternatively, in the case of using the previously known design standard length as an initial estimate, it may be determined at an appropriate ratio such as previously known design error range or 10% of the design error range.
In summary, the range of the solution of Equation (7) is defined based on the initial estimated value of the predetermined absolute length, and the range of the wavelengths within the specified range using Equation (8) as measured from the fractional part (e i), and in theory, ask by the) method conforming the solution of equation (8) in the range that appears to be smaller than a specific phase tolerance (d) differences in the resulting fractional part (e), this is just geometric absolute length .
[3] Principle of Absolute Length Measurement Using Multi-Wavelength Laser Interferometer of the Present Invention
As described in paragraphs [1] and [2] , the geometrical absolute length can be obtained by using a two-wavelength laser interferometer to obtain the compensated geometric length of the medium refractive index and using a multiple laser interferometer. In addition, since the two-wavelength laser interferometer and the multi-laser interferometer have basically similar configurations, it is possible to obtain the geometric absolute length compensated by the medium refractive index by combining them. That is, the multi-wavelength double-laser interferometer of the present invention can be said to be an efficient combination of the two-wavelength laser interferometer and the multi-laser interferometer described in paragraphs [1] and [2] .
FIG. 3 shows an embodiment of an absolute length measuring apparatus using the multiple double-wavelength laser interferometer of the present invention. As shown in FIG. 3, the multi-wavelength laser interferometer of the present invention includes a light source section, a dividing section, a first dichroic mirror, a second dichroic mirror, a light measuring section, and a calculating section. In the following, each part will be described in more detail and how the principles described in paragraphs [1], [2] are applied in the present invention.
Said light source unit, to each other a plurality of elementary light having different wavelengths (λ 1, λ 2, λ 3, λ 4) and a plurality of laser consisting of at least one of the secondary blend Fine Comparison light (λ SHG) is selected from the base light So that light is irradiated toward the object to be measured along the same optical path. In FIG. 3, the portion indicated by [atomic clock + Multi-channel OFGs + SHG generation] is the light source unit. In the multi-channel OFGs, a plurality of base lights (λ 1 , λ 2 , λ 3 , and λ 4 ) having different wavelengths are generated. In the SHG generation, at least one of the base lights is received, (? SHG ). As described in paragraph [2] above, Multi-channel OFGs can be implemented by connecting a plurality of light sources generating specific wavelengths in parallel, and also by considering the descriptions of paragraphs [1] and [2] (λ 1 , λ 2 , λ 3 , λ 4 ) wavelengths are formed with relatively closely distributed values, and the wavelength of the comparison light (λ SHG ) is formed at relatively far apart values. Incidentally, in FIG. 3, the number of basic lights is four, and hence the index value is also expressed as 1, 2, 3, and 4, but the number of basic lights does not necessarily have to be four. As the number of basic lights decreases, the amount of calculations decreases, but the accuracy of calculated absolute length values decreases. As the number of basic lights increases, the accuracy of absolute length values calculated increases as the number of calculations increases, The number of lights can be determined appropriately.
The dividing unit is disposed on the optical path between the light source and the object to divide the light by reflecting part of the light and passing the remaining part. In FIG. 3, three beam splitters denoted by BS1, BS2 and BS3 serve as the light dividing unit. The light reflected by the divided portion becomes reference light, and the light that has passed through the divided portion, reflected by the measured object, and reflected by the divided portion sequentially passes through the measuring light, becomes measurement light. In the example of FIG. 3, Is reflected by the light source, reflected by the BS1, reflected by the BS3, and is reflected by the light source, passes through the BS1, passes through the BS2, reflects from the measured object, The light that travels through it becomes measurement light.
The first dichroic mirror is disposed on the optical path of the reference light and serves to separate the reference-based light and the reference-reference light by reflecting or passing the light according to the wavelength, which is denoted by DM1 in Fig. That is, the first dichroic mirror corresponds to the reference light side
The second dichroic mirror is disposed on the optical path of the measurement light and serves to separate the measurement-based light and the measurement-comparison light by reflecting or passing the light according to the wavelength, which is denoted by DM2 in Fig. That is, the second dichroic mirror corresponds to the measurement light side
The light measuring unit measures reference-based light, reference-comparing light, measurement-based light, and measuring-comparing light reflected or passed through the first dichroic mirror and the second dichroic mirror. In FIG. 3, reference-based light (i.e., light traveling in the order C-BS1-BS3-DM1 and passing through DM1) and measurement-based light (i.e., C-BS1-BS2-RR- ) Is incident on the [PD array] via [FBG array] and measured. Fiber Bragg Grating (FBG) arrays are generally optical components used to amplify optical signals, and PD (photo detector) arrays are optical measurement components in which a number of optical measurement sensors are arranged . BS1 - BS3 - DM1) and measurement - comparison light (ie, C - BS1 - BS2 - RR - DM2 in this order) Reflected light) is incident on the PD provided on each optical path.
The calculation unit compares the reference-based light, the reference-comparison light, the measurement-based light, and the measured-comparison light measured by the optical measuring unit to calculate the geometric absolute length to the measured object. In FIG. 3, [Multi-channel phasemeter + Data process PC], which is connected to both the PD array for acquiring basic light and each PD for acquiring comparison light, is directly calculated.
The calculation unit calculates the geometrical shape of the object to be measured through the method as described in paragraphs [1] and [2] using the measured reference-based light, reference-comparison light, measurement-based light, The absolute length is calculated. The calculations performed by the calculator are briefly summarized as follows.
First, a geometric length in which the medium refractive index is compensated is calculated by using the following equation using at least one of the fundamental lights and the comparison light (see paragraph [1] , descriptions of
(From here,
L: geometrical length to the object to be measured,
D i is the optical path length measured by the i-th basic light wavelength (? I ) (the fundamental light which is the source of the comparison light in which the i-th basic light is the secondary harmonic light)
D SHG is the optical path length measured by the comparative light wavelength? SHG ,
A: amplification factor,
Also, the geometric absolute length is calculated by applying the summation method which defines the range of the solution (L c ) to a predetermined initial estimate in the following equation (see paragraph [2] , explanation of equations 7 and 8).
(From here,
i: index of the fundamental light,
N: the total number of the base lights,
m i : i < th > fundamental light phase integral part,
e i : i th basic optical phase fractional part,
L c : the year determined by the concatenation,
d: phase tolerance)
I.e. In sum, first, but yields a two-wavelength laser interferometer medium refractive index using the principle compensating the geometrical length, this method the i-th medium, the refractive index is the geometric length that is L i compensation to apply to the i of elementary light i more (this will each include a wavelength λ i, m i the integer part, decimal part e i, the integer part of m i being only unknowns in) can be obtained. Next, geometric absolute lengths can be obtained by using the geometric length i obtained in this way and using the summation method. In this case, since the refractive index of the medium has already been compensated in the previous step, the geometrical absolute length obtained by the congruent method as a result is the compensated geometrical absolute length of the medium refractive index.
Meanwhile, in order to increase the accuracy in the process of obtaining the geometric absolute length by the above-described method, it is also possible to do the following. In order to obtain the compensated geometric length of the medium refractive index, the following equation is used.
The L obtained in this way can be said to be actually L i when considering that the subscript i represents the basic light index, and L in the summation formula is actually L i . For example, the geometric length L 1 in which the refractive index of the medium is compensated by using the base light having the wavelength of 1 is obtained, and the solution is obtained by placing this L 1 on the left side of the summation formula.
This method is repeated for each basic light, that is, the number N of basic lights, and the geometric absolute length L 1 , , Li , ... , L N can be obtained. That is, the geometric length Li compensated for the refractive index of the medium is calculated for the i-th basic light using the following equation,
(Wherein, L i: the i-th geometrical length, D i to said measured object measured by the base light: the i-th basis of the wavelength of light (the optical path length (a i based on the light measured by the λ i) is the secondary blend D SHG : optical path length measured by the comparative light wavelength (? SHG ), A: amplification factor,
Next, the medium refractive index to the measured object measured by the i < th > base light is compensated using the compensated geometrical length (Li), but the range of the solution ( Lc ) is limited to the initial estimate of the predetermined geometric absolute length The geometric absolute length measured by the i < th > base light is calculated by applying the summation method,
(Where, i: it is sought to combine method: An index of the base light, N: total number of the base light, m i: the i-th base optical phase integer, e i: i-th basic optical phase fractional part, L c , d: phase tolerance)
Finally, the geometric absolute length L is calculated by averaging the compensated geometric absolute length (Li) of the medium refractive indexes obtained for each of the N i-th basis light beams by the following equation.
4 is another embodiment of the absolute length measuring apparatus using the multiple double-wavelength laser interferometer of the present invention. 4, a plurality of basic light beams (? 1 ,? 2 ,? 3 ,? 4 ) and comparison light beams? SHG emitted from the light source unit are further separated and passed through an acoustooptic modulator (AOM) And further incident on the optical system of Fig. 3 using BS4. An acoustooptic modulator is an optical element that shifts the frequency of light passing through it. It is a device used to measure the phase more precisely by constructing a heterodyne interferometer.
The role of the acousto-optic modulator, AOM, will be described in more detail as follows. Generally, optical signals are measured by a photodetector (PD), and the optical measuring units in the above description are also made up of PDs as shown in FIGS. 3 and 4. That is, the above description assumes that the phase information of the laser light is obtained from the optical signal measured by the PD. However, in practice, it is very difficult to directly measure the phase information when the frequency of the laser light used for the length measurement is very high. Therefore, the heterodyne technique is used to keep the phase information as it is but to lower the frequency information of the optical frequency, and the AOM is provided for the application of this technique.
Specifically, if a frequency shift is performed using a known frequency interval using the AOM and interference is generated, a sine wave having a frequency difference equal to the frequency interval applied by the AOM can be obtained in the photodetector. At this time, the phase information of the sine wave has the phase information of the original optical frequency. The equation is expressed as follows.
Measuring light:
AOM shift light:
Measurement light * AOM shift Optical interference:
In the above equation, the photodetector is an electronic device
[4] Principle of wavelength stabilization in light source
Hereinafter, an embodiment, a principle, and a technical meaning of a light source capable of generating a stabilized wavelength laser light as described above will be described. FIGS. 5 and 6 show embodiments of the stabilized wavelength generation light source configuration, and FIGS. 7 and 8 are views for explaining the respective principles. 9 is an embodiment of a light source configuration capable of generating basic light-comparison light.
As described above, it is possible to measure the absolute length by eliminating the influence of environmental variables by measuring the length by using a double-wavelength laser interferometer, but there is a precondition that the wavelength should be stabilized to an accurate value, There is a problem that the error is amplified. Specifically, in Equation (4), the geometric length L is obtained by adding the amplification factor A to the difference (D SHG - D) between the optical path lengths D SHG and D measured at the comparison light wavelength at the optical path length D measured at the base light wavelength Subtracted by the multiplied value. In this case, as described above, the amplification factor A is 141.41, that is, a constant value in the order of 10 2 when the wavelength is 1555 nm or 777.5 nm when the light source is used. In other words, if there is an error in the value (D SHG - D), there is a problem that the error can be amplified 100 times. Also, in calculating the absolute length using a plurality of basic light beams, if the wavelength value is not correct, an error occurs in the result.
In order to solve such an error amplification or accumulation problem, in the present invention, the light source unit includes an optical comb and an external laser, and a laser generated from the external laser using the frequency of the laser light generated in the optical comb So that the frequency of the light is stabilized to a predetermined frequency.
The optical comb is a device for generating a laser beam having a plurality of reference frequencies spaced apart from each other at a predetermined interval in the frequency domain, and is widely used in various optical fields. Since the comb can generate lights with frequencies of a certain frequency band very stably, there is a great potential for applications such as the application of the combs won the 2005 Nobel Prize in Physics. However, there is a limit that the amount of light generated from such an optical comb itself is considerably smaller than the amount of laser light used for measurement purposes.
The external laser is a laser generally used for measurement or the like. 5 to 8 show an example of using a DFB laser (distributed feedback laser) as the external laser. The DFB laser is a laser having a resonator having a wavelength selectivity by allowing an optical waveguide to have a periodic structure. The laser has the same light emission principle as that of an ordinary semiconductor laser, but has a concavo-convex structure in the light- So that the desired wavelength can be selectively oscillated. Although DFB lasers have the advantage of generating more stable laser light than semiconductor lasers, DFB lasers have a wide line width of several MHz, so that the frequency stability and accuracy of the laser light generated by the DFB lasers is still not sufficiently stable . Such a change in the laser light frequency negatively affects the measurement accuracy, which is a cause of the optical path length error described above.
In the present invention, by connecting an external laser having a sufficient quantity of light to be used for measurement to the optical comb, the frequency of the laser light generated in the external laser is determined at a predetermined frequency To be locked. Thus, by combining the light amount of the external laser and the stability of the optical comb, the optical path length error as described above is minimized by generating the laser light having the stabilized frequency, and ultimately, the accuracy of the finally calculated geometric length is improved It becomes possible to maximize it.
There are various methods of stabilizing the frequency by connecting the external combiner and the comb which constitute the light source unit. FIG. 5 shows a method of using a phase locked loop (PLL) And FIG. 6 shows an injection locking method according to another embodiment of the present invention. However, the configuration of the light source unit is not limited to the configuration shown in FIG. 5 or 6, and other methods may be employed as long as the frequency of the external laser can be stabilized using a light comb. In addition to the above two optical frequency generators (based on the phase lock circuit, based on the injection locking method), a monochromatic laser stabilized in the frequency standard can be used as the light source. Each embodiment will be described in more detail below.
Fig. 7 shows the principle of the PLL scheme, which is an example of the stabilization wavelength generation principle, that is, the example shown in Fig. 5, the light source unit further includes an atomic clock connected to the optical comb and a phase locked loop (PLL) connected to the atomic clock, The external laser is connected to the phase lock circuit so that the laser light generated in the comb and the laser light generated in the external laser are synchronized to stabilize the frequency of the laser light generated in the external laser.
The graph shown on the upper side of FIG. 7 shows the shape of the optical signal generated in the optical comb. As shown in the figure, a laser beam having a plurality of reference frequencies spaced apart from each other at regular intervals (indicated by a thick solid line in the upper graph of FIG. 7) is generated in the optical comb. Among these reference frequencies, a stabilization reference frequency for stabilizing the frequency of the external laser is selected (the criterion for such selection can be appropriately determined according to the user's purpose or necessity). The stabilization reference frequency value thus selected can be expressed as shown in Equation (9) as can be seen from the graph of FIG. 7 intuitively.
&Quot; (9) "
f i = if r + f o
(From here,
f i is a stabilization reference frequency selected for stabilizing the external laser light frequency among reference frequencies of the laser light generated in the light comb,
f r is a repetition rate of laser light generated in the light comb,
if r : the i-th repetition rate (i is a natural number) closest to the stabilization reference frequency among the repetition rate values smaller than the stabilization reference frequency,
f o is an offset frequency between the stabilization reference frequency and the i th repetition rate,
The optical comb is connected to the atomic clock, and the laser light of the optical comb having the reference frequency for stabilization is synchronized with the atomic clock. The phase lock circuit is connected to the atomic clock, and the external laser is connected to the phase lock circuit. That is, the external laser connected through the optical comb and the phase lock circuit by the atomic clock is synchronized, so that the frequency of the external laser can be locked. At this time, there may be a slight difference between the external laser and the reference frequency for stabilization. Assuming that the difference is a bit frequency, the frequency value of the external laser stabilized as described above is also shown in the graph of FIG. 7 As can be seen intuitively from the very beginning, it can be expressed as Equation 10 below.
&Quot; (10) "
f DFB = if r + f o + f b
(From here,
f DFB is a frequency of laser light generated in the external laser,
f b is a bit frequency between the reference frequency for stabilization and the frequency of laser light generated by the external laser)
Fig. 8 shows the principle of an injection locking method, which is an alternative embodiment of the stabilization wavelength generation principle, that is, the example shown in Fig. 6, the light source unit includes an atomic clock connected to the optical comb, a Fabry-Perot filter connected to the optical comb, (FBG) connected to the Faro filter. The laser beam emitted from the comb passes through the filter unit, so that only a part of the modes is selected. And the frequency of the laser light generated by the external laser is stabilized by being incident on the external laser by the circulator.
In the injection locking method, the atomic clock and the optical comb play the same role as in the PLL method, and thus the graph of the optical signal shape generated in the optical comb shown in the upper side of FIG. 8 is the same as the graph on the upper side of FIG. Therefore, even in the case of the injection locking method, the reference frequency value for stabilization can be obtained using Equation (9).
On the other hand, in the PLL system, an optical comb and a phase lock circuit are connected in parallel to the atomic clock, and an external laser is connected to the phase lock circuit. That is, , A gap may be generated between the stabilization reference frequency selected from the optical comb and the frequency of the external laser. On the other hand, in the injection locking method, since the optical comb and the filter unit are connected to the atomic clock and the external laser is connected to the output end of the atomic clock, the frequency of the external laser is maintained It is stabilized to be equal to the reference frequency. That is, in the injection locking method, the frequency value of the stabilized external laser can be expressed as shown in Equation (11) as can be seen intuitively from the graph shown in FIG.
&Quot; (11) "
f DFB = if r + f o
(From here,
f DFB : frequency of laser light generated from the external laser)
In summary, as described above, in the length measuring apparatus of the present invention, the light source unit is configured to include an optical comb and an external laser, so that the laser light generated in the light source unit has a stabilized frequency, Thereby maximizing the accuracy of the geometric length that is ultimately calculated.
The laser light generated by using the optical comb and the external laser has a stabilized wavelength and frequency. 9 illustrates an embodiment of the basic light-comparison light generating light source, which is similar to that of FIG. 5 or 6 except that the light source for generating the stabilizing wavelength is separated from the light source for generating the base light and the separation light The configuration is disclosed. In the example of FIG. 9, the laser light is divided into one and used as it is, and the other is made to pass the PPLN (Periodically Poled Lithium Niobate) generating the second harmonic wave to fold the wavelength in half. This will be described in more detail as follows.
As shown in FIG. 5 or 6, the light source unit includes an optical coupler (OC) that divides the laser light emitted from a light source that generates a stabilized wavelength, and generates a second harmonic wave of the incident light. A periodically poled lithium niobate (PPLN) that folds in half, and a dichroic mirror (DM) that transmits or reflects light according to wavelength.
At this time, one of the lights split by the optical coupler is made to pass through the dichroic mirror toward the dichroic mirror, and this light becomes the base light (?). On the other hand, the other one of the lights split by the optical coupler passes through the PPLN and then proceeds toward the dichroic mirror and is reflected by the dichroic mirror so that the light passing through the dichroic mirror and the light reflected from the dichroic mirror are the same The light is made to proceed to the optical path, and this light becomes the comparison light ( SHG ). That is, through the apparatus as shown in FIG. 9, light emitted from the light source unit can be formed to be composed of the base light? And the comparison light? SHG .
It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. It goes without saying that various modifications can be made.
510: Two-wavelength laser interferometer
511: (two-wavelength laser interferometer)
512: (two-wavelength laser interferometer) splitter
513r: (Two-wavelength laser interferometer) Dichroic mirror on the reference light side
513m: (two-wavelength laser interferometer)
514r: (two-wavelength laser interferometer) reference -
514rs: (double wavelength laser interferometer) Reference -
514m: (two-wavelength laser interferometer) measurement-based optical measuring unit
514ms: (two-wavelength laser interferometer) measurement-comparison light measuring part
520: multiple laser interferometer
521: (multiple laser interferometer)
522: (multiple laser interferometer) splitting section
523r: (multiple laser interferometer) reference light measuring unit
523m: (multiple laser interferometer) measuring light measuring part
Claims (10)
A splitting unit disposed on an optical path between the light source and the object to divide the light by reflecting part of the light and passing the remaining part;
When the light reflected from the divided portion is referred to as a reference light and the light that has passed through the divided portion, reflected by the measured object, and reflected sequentially by the divided portion is referred to as measuring light,
A first dichroic mirror disposed on the optical path of the reference light for separating the reference-based light and the reference-reference light by reflecting or passing the light according to the wavelength;
A second dichroic mirror disposed on the optical path of the measurement light for separating the measurement-based light and the measurement-comparison light by reflecting or passing the light according to the wavelength;
A light measuring unit for measuring reference-based light, reference-comparing light, measurement-based light, and measurement-comparison light reflected or passed through the first dichroic mirror and the second dichroic mirror;
A calculation unit for calculating a geometric absolute length to the object by comparing the reference-base light, the reference-comparison light, the measurement-basis light, and the measurement-comparison light measured by the light measuring unit;
And a second laser interferometer for measuring the absolute length of the laser beam.
A plurality of base lights (? 1 ,? 2 ,? 3 ,? 4 ) and a comparison light (? SHG )
Wherein the geometric length (L) of the medium refractive index is compensated by using the following equation: < EMI ID = 1.0 >
(From here,
L: geometrical length to the object to be measured,
D i is the optical path length measured by the i-th basic light wavelength (? I ) (the fundamental light which is the source of the comparison light in which the i-th basic light is the secondary harmonic light)
D SHG is the optical path length measured by the comparative light wavelength? SHG ,
A: amplification factor,
A plurality of base lights (? 1 ,? 2 ,? 3 ,? 4 )
Wherein the geometric absolute length is calculated by applying a summation method according to the following equation that defines the range of the solution (L c ) as an initial estimate of a predetermined geometric absolute length: .
(From here,
i: index of the fundamental light,
N: the total number of the base lights,
m i : i < th > fundamental light phase integral part,
e i : i th basic optical phase fractional part,
L c : the year determined by the concatenation,
d: phase tolerance)
A plurality of base lights (? 1 ,? 2 ,? 3 ,? 4 ) and a comparison light (? SHG )
The geometric length Li compensated for the refractive index of the medium is calculated using the following equation for the i < th > base light,
(From here,
L i is the geometrical length to the measured object measured by the i-th basis light,
D i is the optical path length measured by the i-th basic light wavelength (? I ) (the fundamental light which is the source of the comparison light in which the i-th basic light is the secondary harmonic light)
D SHG is the optical path length measured by the comparative light wavelength? SHG ,
A: amplification factor,
Using a geometric length (Li) compensated for the medium refractive index to the measured object by the i-th basis light, and defining a range of the solution (L c ) as an initial estimate of the predetermined geometric absolute length To calculate the geometric absolute length measured by the i < th > base light,
(From here,
i: index of the fundamental light,
N: the total number of the base lights,
m i : i < th > fundamental light phase integral part,
e i : i th basic optical phase fractional part,
L c : the year determined by the concatenation,
d: phase tolerance)
Characterized in that a geometric absolute length (Li) compensated for the medium refractive index, which is obtained for each of the N first base light beams, is averaged by the following equation to calculate the geometrical absolute length L in which the medium refractive index is compensated. For measuring the absolute length.
An optical comb for generating a laser beam having a plurality of reference frequencies spaced apart from each other at regular intervals in the frequency domain, and an external laser,
And the frequency of the laser light generated in the external laser is stabilized at a predetermined frequency by using the frequency of the laser light generated in the optical comb.
An atomic clock connected to the optical comb, and a phase locked loop (PLL) connected to the atomic clock, the external laser being connected to the phase lock circuit,
Wherein the laser light generated in the optical comb and the laser light generated in the external laser are synchronized so that the frequency of the laser light generated in the external laser is stabilized.
Wherein the frequency of the laser light generated by the external laser is stabilized according to the following equation.
f i = if r + f o
f DFB = if r + f o + f b
(From here,
f DFB is a frequency of laser light generated in the external laser,
f i is a stabilization reference frequency selected for stabilizing the external laser light frequency among reference frequencies of the laser light generated in the light comb,
f r is a repetition rate of laser light generated in the light comb,
if r : the i-th repetition rate (i is a natural number) closest to the stabilization reference frequency among the repetition rate values smaller than the stabilization reference frequency,
f o is an offset frequency between the stabilization reference frequency and the i th repetition rate,
f b is a bit frequency between the reference frequency for stabilization and the frequency of laser light generated by the external laser)
An atomic clock connected to the optical comb, a Fabry-Perot filter connected to the optical comb, and a fiber Bragg grating (FBG) connected to the Fabry-Perot filter, Lt; / RTI >
The laser light generated in the optical comb passes through the filter unit, and only a partial mode is selected. When the laser light selected in the partial mode is incident on the external laser by the circulator, the frequency of the laser light generated in the external laser is stabilized Wherein the first and second laser interferometers are formed in the same direction.
Wherein the frequency of the laser light generated by the external laser is stabilized according to the following equation.
f i = if r + f o
f DFB = if r + f o
(From here,
f DFB is a frequency of laser light generated in the external laser,
f i is a stabilization reference frequency selected for stabilizing the external laser light frequency among reference frequencies of the laser light generated in the light comb,
f r is a repetition rate of laser light generated in the light comb,
if r : the i-th repetition rate (i is a natural number) closest to the stabilization reference frequency among the repetition rate values smaller than the stabilization reference frequency,
f o is an offset frequency between the stabilization reference frequency and the i th repetition rate,
OC (Optic Coupler) for dividing and advancing the laser beam, PPLN (Periodically Poled Lithium Niobate) for folding the wavelength of light passing through by generating a second harmonic wave of the incident light, (DM, Dichroic Mirror) which reflects the light,
Wherein one of the lights split by the optical coupler travels toward the dichroic mirror to pass through the dichroic mirror to form a base light and the other of the lights split by the optical coupler passes through the PPLN, And the reference light and the comparison light proceed to the same light path, and the reference light and the comparison light proceed to the same light path,
Wherein the light emitted from the light source unit is formed to have a fundamental light wavelength and a comparative light wavelength.
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| WO2005015149A1 (en) | 2003-08-12 | 2005-02-17 | Bussan Nanotech Research Institute, Inc. | Detection device, optical path length measurement device, measurement instrument, optical member evaluation method, and temperature change detection method |
| KR101198013B1 (en) | 2011-08-22 | 2012-11-07 | 서강대학교산학협력단 | Multi-wavelength heterodyne interferometer using AOTF |
| KR101642952B1 (en) | 2015-11-11 | 2016-07-26 | 한국과학기술원 | Graphical length measuring apparatus using two-color interferometer |
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| WO2005015149A1 (en) | 2003-08-12 | 2005-02-17 | Bussan Nanotech Research Institute, Inc. | Detection device, optical path length measurement device, measurement instrument, optical member evaluation method, and temperature change detection method |
| KR101198013B1 (en) | 2011-08-22 | 2012-11-07 | 서강대학교산학협력단 | Multi-wavelength heterodyne interferometer using AOTF |
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