WO2017133128A1 - 一种大范围绝对距离测量中空气折射率的修正方法 - Google Patents

一种大范围绝对距离测量中空气折射率的修正方法 Download PDF

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WO2017133128A1
WO2017133128A1 PCT/CN2016/082279 CN2016082279W WO2017133128A1 WO 2017133128 A1 WO2017133128 A1 WO 2017133128A1 CN 2016082279 W CN2016082279 W CN 2016082279W WO 2017133128 A1 WO2017133128 A1 WO 2017133128A1
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wavelength
laser
refractive index
measurement
absolute distance
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French (fr)
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陈本永
张世华
严利平
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浙江理工大学
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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
    • 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
    • G01B9/02002—Interferometers characterised by controlling or generating intrinsic radiation properties using two or more frequencies
    • G01B9/02005—Interferometers characterised by controlling or generating intrinsic radiation properties using two or more frequencies using discrete frequency stepping or switching
    • 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
    • G01B9/02008—Two or more frequencies or sources used for interferometric measurement by using a frequency comb
    • 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
    • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
    • G01B9/02062—Active error reduction, i.e. varying with time
    • 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
    • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
    • G01B9/02075—Reduction or prevention of errors; Testing; Calibration of particular errors
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B2290/00—Aspects of interferometers not specifically covered by any group under G01B9/02
    • G01B2290/60—Reference interferometer, i.e. additional interferometer not interacting with object

Definitions

  • the invention relates to a method for correcting the refractive index of air in absolute distance measurement, in particular to a method for correcting the refractive index of air in a wide range of absolute distance measurement.
  • the scale of the laser interferometry distance measurement is the laser wavelength, which is equal to the product of the laser vacuum wavelength and the refractive index of the air.
  • the uncertainty of laser interferometry is mainly derived from the uncertainty of air refractive index measurement, especially in the absolute range of interferometry (refer to the absolute distance to be measured is more than 1m) absolute distance measurement, the achievable ranging accuracy is generally It is not limited by the stability of the laser wavelength, but depends on the accurate measurement of the environmental parameters. To get a more accurate measurement result, the real-time air refractive index along the laser beam path must be known.
  • the commonly used air refractive index measurement and compensation method is the PTF method, that is, the temperature, pressure, humidity and concentration of CO 2 in the air measured by the sensor, and the refractive index of the air is calculated according to the Edlén formula, and the temperature can be well controlled. Under laboratory conditions, measurement accuracy of 1 ⁇ 10 -8 can be achieved. However, the accuracy of this method is limited by the instability and non-uniformity of air quality. In general laboratory and workshop environments, temperature changes and gradations are faster and more local than other parameters. Pressure and humidity changes need to be considered in extreme industrial applications with strong air disturbances. The temperature measurement in the traditional air refractive index measurement method is only reflected in a single point, several points or a line adjacent to the optical path.
  • the refractive index of the air measured by the refractometer is also the refractive index value at a given time and at a given location. It is not the laser interferometer that measures the refractive index of the air over the entire optical path. Therefore, it is difficult to correct the absolute distance to be measured along the precise measuring optical path by using the conventional air refractive index measurement and compensation method due to the temporal variation and spatial distribution of the refractive index of the air.
  • the two-color method is a self-correcting method for realizing the refractive index of air by length measurement using lasers of different colors. It uses the dispersion relation of the refractive index of air, so that the influence of temperature and air pressure on the accuracy of ranging is greatly reduced, and it is almost negligible.
  • the range measurement results are only slightly affected by the water vapor in the air, and in many cases the humidity measurement can achieve sufficient accuracy. Therefore, the two-color method is an effective method to realize the self-correction of the full refractive index of the air in the wide range absolute distance measurement.
  • the key to the two-color air refractive index correction is the selection of two laser wavelengths that determine the magnitude of the dispersion coefficient and the high-precision measurement of the optical path difference between the two wavelengths. Since the measurement uncertainty of the optical path difference corresponding to the two wavelengths is amplified by the dispersion coefficient, thereby affecting the final distance correction accuracy, the smaller the dispersion coefficient, the more Conducive to improve the correction accuracy, which requires two laser wavelengths with large wavelength difference, usually using two wavelengths with frequency doubling (or triple frequency) relationship (for example, one is the visible wavelength and the other is the near-infrared wavelength) Performing a two-color method reduces the dispersion coefficient, thereby reducing the effect of laser wavelength uncertainty or drift on measurement accuracy.
  • the chromatic aberration caused by the wide interval between the two laser wavelengths with a large wavelength difference introduces a large error, and special optical devices such as a lens and a beam splitter are required to reduce the influence of chromatic aberration, which increases the cost of the optical system, and
  • special optical devices such as a lens and a beam splitter are required to reduce the influence of chromatic aberration, which increases the cost of the optical system.
  • the wavelength difference between the two color lights is large, the laser intensity caused by the air disturbance in the measurement environment and the phase fluctuation of the interference signal will also increase the measurement uncertainty of the optical path difference corresponding to the two wavelengths, thereby affecting the distance correction accuracy.
  • the present invention provides a method for correcting the refractive index of air in a wide range of absolute distance measurement, and adopts a two-color method based on laser single wavelength and synthetic wavelength to realize self-correction of air refractive index in absolute distance measurement.
  • An accurate compensation method for the refractive index of the air is provided for a wide range of absolute distance measurements.
  • the first laser outputs a laser beam fixed at a single wavelength ⁇ 1
  • the second laser sequentially outputs a laser beam whose wavelength is from small to large, that is, ⁇ 21 ⁇ 22 ⁇ ... ⁇ 2i ⁇ ... ⁇ 2n , two laser beams
  • orthogonal linearly polarized light is formed, and enters a laser interference system composed of a beam splitter, a second polarization beam splitter, a first corner cube prism and a second corner cube prism;
  • Half of the first-level synthesis wavelength ⁇ S1 is greater than the optical distance L corresponding to the absolute distance D to be measured, that is, ⁇ S1 /2>L should be satisfied, and the minimum synthesis wavelength ⁇ Sn should be transitioned between levels to a single wavelength ⁇ 1 That requires u(L Sn ) ⁇ 1 /4;
  • a S1 (n Sn -1) / (n Sn - n 1 ), A S1 is the dispersion coefficient corresponding to ⁇ Sn and ⁇ 1 ; n Sn and n 1 are the refractive indices of air corresponding to ⁇ Sn and ⁇ 1 , respectively. Calculated by the Edlén formula.
  • the wavelengths of the first laser and the second laser output laser belong to the same spectral band, and the minimum synthesized wavelength ⁇ Sn is constructed by two single wavelengths ⁇ 1 and ⁇ 2n of the same spectral band.
  • the present invention has the following beneficial effects:
  • the two laser wavelengths ⁇ 2n and ⁇ 1 used to construct the minimum synthesized wavelength ⁇ Sn belong to the same spectral band, and their wavelength values are close, which effectively reduces the laser intensity and phase shift of the interference signal caused by air disturbance in the measurement environment.
  • the influence on the accuracy of the distance correction avoids the use of special lenses and special beam splitters that eliminate the influence of chromatic aberration, which reduces the complexity and cost of the optical system.
  • the invention has the advantages of large ranging range and high distance correction precision, and belongs to the full path of the refractive index of the air along the actual measuring optical path, and has low requirements for measuring the environmental parameters such as temperature and pressure, and is easy to implement.
  • Figure 1 is a schematic diagram of a method for correcting the refractive index of air in a wide range of absolute distance measurements.
  • FIG. 1 it is a schematic diagram corresponding to an embodiment of the present invention, including a light source system I, a laser interference system II and a signal processing system III.
  • the specific implementation process is as follows:
  • the light source system 1 is composed of a first laser 1, a second laser 2, a mirror 3 and a first partial polarizer 4.
  • the first laser 1 uses a tunable semiconductor laser to output a light beam with a single wavelength ⁇ 1 fixed and a polarization direction parallel to the paper surface;
  • the second laser 2 uses another tunable semiconductor laser with an adjustable output wavelength ⁇ 2 and a polarization direction
  • the light beam perpendicular to the paper surface adjusts the value of the wavelength ⁇ 2 by changing the driving voltage and the operating current of the piezoelectric ceramic;
  • the light beam output from the first laser 1 is directly directed to the first polarization beam splitter 4, and the output of the second laser 2
  • the light beam is reflected by the mirror 3 and then directed to the first polarization beam splitter 4, which are respectively transmitted and reflected by the first polarization beam splitter 4 to form a bundle of orthogonal linearly polarized light.
  • the laser interference system II is composed of a beam splitter 5, a second polarization beam splitter 6, a first corner cube prism 7, and a second corner cube prism 8. After the orthogonal linearly polarized light from the light source system I is directed to the beam splitter 5, the reflected light and the transmitted light are used as a reference beam and a measuring beam, respectively.
  • the beam of wavelength ⁇ 2 is reflected by the second polarization beam splitter 6 and directed toward the beam splitter 5; the beam of wavelength ⁇ 1 is transmitted through the second polarization beam splitter 6, After being reflected by the first corner cube prism 7, it returns, passes through the second polarization beam splitter 6 again, and is incident on the beam splitter 5.
  • the measuring beam is reflected by the second cube-corner prism 8 and returned to the beam splitter 5, and after being reflected by the beam splitter 5, it merges with the reference beam transmitted through the beam splitter 5 and interferes.
  • the signal processing system III is composed of a third polarization beam splitter 9, a first photodetector 10, a second photodetector 11, an interference signal processing module 12, and a computer 13.
  • the interference beam from the laser interference system II is directed to the third polarization beam splitter 9, wherein the interference beam of wavelength ⁇ 1 is transmitted through the third polarization beam splitter 9 and received by the first photodetector 10 at a wavelength of ⁇ 2
  • the interference beam is reflected by the third polarization beam splitter 9, it is received by the second photodetector 11.
  • the interference signals output by the two photodetectors are processed by the interference signal processing module 12 and transmitted to the computer 13, and the two-color air is calculated by the computer 13. Refractive index correction distance.
  • the laser interfering system of the present invention can be applied to a measuring device similar to that in the Chinese Patent Application Publication No. Hei. No. Hei. No. Hei. No. Hei.
  • the first laser 1 adopts a TLB 6704 tunable semiconductor laser of Newport Corporation of the United States, and the fixed single wavelength ⁇ 1 is 631 nm, and the second laser 2 is tunable by a DL Pro 633 type of Toptica of Germany.
  • the output wavelength ⁇ 2 is adjusted from 630 nm to 637 nm
  • the first photodetector 10 and the second photodetector 11 are two S09105 PIN photodetectors
  • the interference signal processing module 12 is SIS3316 of the German Struck company.
  • a high-speed data acquisition card, the computer 13 uses a Hewlett-Packard Pro4500 desktop computer.
  • the optical path of the imaginary vertical line position to the beam splitter 5 is equal to the optical path of the second polarization beam splitter 6 to the beam splitter 5, and the virtual vertical line is recorded.
  • the position is the measurement starting point;
  • the absolute distance D to be measured is the geometric distance between the measurement starting point and the second corner cube prism 8;
  • the first laser 1 outputs linearly polarized light of a single wavelength ⁇ 1 ; the second laser 2 outputs linearly polarized light of adjustable wavelength ⁇ 2 , and adjusts the piezoelectric ceramic driving voltage and operating current of the second laser 2, first outputting The wavelength is ⁇ 21 , and the half of the first-order synthesis wavelength ⁇ S1 formed by the wavelength ⁇ 21 and the wavelength ⁇ 1 is greater than the optical distance L corresponding to the absolute distance D to be tested;
  • the second corner cube prism 8 is placed at the measurement starting point, and the first corner cube prism 7 is adjusted so that the interference signal phases corresponding to the wavelengths ⁇ 1 and ⁇ 21 received by the first photodetector 10 and the second photodetector 11 are Zero crossing at the same time, the phase difference between the two Note that the position of the first corner cube 7 is the reference starting point;
  • the integer interference fringe number M Si of ⁇ Si is calculated by the computer 13 according to the following formula:
  • M 1 represents a single wavelength ⁇ integer number of interference fringes
  • ⁇ 1 represents a single wavelength ⁇ 1 of the fractional interference fringes
  • ⁇ L Si represents the optical distance corresponding to the fractional interference fringe of the minimum combined wavelength ⁇ Sn
  • ⁇ l n represents the optical distance corresponding to the displacement ⁇ d n of the first corner cube 7 .
  • the interference fringes corresponding to ⁇ 1 and ⁇ Sn are equal, that is:
  • ⁇ 1 represents the interference fringe fraction of the single wavelength ⁇ 1
  • ⁇ Sn represents the interference fringe fraction of the minimum synthesized wavelength ⁇ Sn .
  • optical distances L Sn and L 1 measured by the minimum combined wavelength ⁇ Sn and the single wavelength ⁇ 1 are directly used for the correction of the refractive index of the two-color method, and the corrected absolute distance is obtained:
  • a S1 (n Sn -1) / (n Sn -n 1 ) is the dispersion coefficient corresponding to ⁇ Sn and ⁇ 1
  • n Sn is the group refractive index corresponding to ⁇ Sn :
  • n 1 and n 2 are air phase refractive indices corresponding to ⁇ 1 and ⁇ 2n , respectively, which are calculated by the Edlén formula;
  • the corrected absolute distance D is calculated by the computer 13 according to the formula (11).
  • the present invention adopts a two-color method based on laser single wavelength and synthetic wavelength to realize absolute distance measurement of air refractive index self-correction.
  • the degree of certainty is:
  • the present invention provides a method for correcting the refractive index of air in a wide range of absolute distance measurement, and the relative uncertainty of the absolute distance measurement in the measurement range of 100 m is 2.5 ⁇ 10 -8 .
  • This method can reduce the influence of air disturbance in the measurement environment.
  • the full path compensation of the air refractive index is realized, and the correction precision is high, the application range is wide, and it is easy to implement.

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  • General Physics & Mathematics (AREA)
  • Instruments For Measurement Of Length By Optical Means (AREA)
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Abstract

一种大范围绝对距离（D）测量中空气折射率的修正方法。采用基于激光单波长（λ 1）与合成波长的双色法实现绝对距离（D）测量中空气折射率的自我修正。两个激光器（1,2）分别输出波长固定和波长可调的激光光束，经激光干涉***（II）构建从大到小的合成波长链（λ S1，…，λ Si，…，λ Sn），每级合成波长之间满足级间过渡条件，且最小合成波长（λ Sn）能过渡至单波长（λ 1），分别对待测的绝对距离（D）进行测量，得到精度逐渐提高的光学距离估计值（L S1，…，L Si，…，L Sn）；最小合成波长（λ Sn）和单波长（λ 1）对应的光学距离（L Sn，L 1）同时得到，根据双色法空气折射率修正原理得到修正后的绝对距离（D）。本方法适用的测距范围大、距离修正精度高，属于沿着实际测量光路的空气折射率全路径修正，且对温度、压强等环境参数的测量精度要求低，易于实现。

Description

一种大范围绝对距离测量中空气折射率的修正方法 技术领域
本发明涉及绝对距离测量中空气折射率的修正方法，尤其涉及一种大范围绝对距离测量中空气折射率的修正方法。
背景技术
激光干涉法距离测量的刻度尺是激光波长，此波长值等于激光真空波长和空气折射率的乘积。激光干涉测距的不确定度主要来源于空气折射率测量的不确定度，特别是在干涉法大范围(指待测的绝对距离大于1m以上)绝对距离测量中，可实现的测距精度一般不受激光波长稳定性的限制，而是依赖于环境参数的精确测量，要得到更精确的测距结果，必须知道沿激光光路方向的实时空气折射率。常用的空气折射率测量和补偿方法是PTF法，即通过传感器测得周围环境的温度、压强、湿度和空气中CO2的浓度，根据Edlén公式计算得到空气折射率，在温度能得到良好控制的实验室条件下，能达到1×10-8的测量精度。但这种方法的精确度会因空气质量的不稳定和不均匀而受到限制，在一般的实验室和车间环境下，温度的改变和渐变比其他参数的改变更快、更具有局部性，在有较强空气扰动的极端工业应用环境下，压强和湿度的变化也需要考虑。传统的空气折射率测量方法中温度的测量仅仅体现在单点、几个点或邻近光路的一条线上，通过一个或几个温度传感器很难探测温度梯度，且温度传感器的反应时间也比较长，从几秒到几分钟，快速的温度改变不能被及时的检测到。通过折射率计测出的空气折射率也只是特定时间和给定地点的折射率值，并不是激光干涉仪进行距离测量整条光路上的空气折射率。因此由于空气折射率的时间变化和空间分布，使用传统的空气折射率测量与补偿方法沿着精确的测量光路对待测的绝对距离进行修正是难以实现的。
双色法是采用不同颜色的激光通过长度测量实现空气折射率的自我修正的方法，它运用了空气折射率的色散关系，使得温度与气压对测距精度的影响大大减小，几乎可以忽略不计，测距结果只略受空气中水蒸气的影响，而在很多情况下湿度测量都可以达到足够的精度，因此双色法是实现大范围绝对距离测量中空气折射率全路径自我修正的有效方法。
双色法空气折射率修正的关键是：决定色散系数大小的两个激光波长的选择和两个波长对应光程差的高精度测量。由于两个波长对应光程差的测量不确定度会被色散系数放大，从而影响最终的距离修正精度，故色散系数越小越有 利于提高修正精度，这就需要有较大波长差的两个激光波长，通常使用两个具有倍频(或三倍频)关系的波长(例如一个是可见光波长，另一个是近红外光波长)进行双色法测量，可降低色散系数，从而降低激光的波长不确定度或漂移对测量精度带来的影响。但较大波长差的两个激光波长之间的宽间隔引起的色差会引入较大误差，而且需要特殊的透镜和分光镜等光学器件来降低色差的影响，这使得光学***的造价提高，且两色光的波长差较大时测量环境中空气扰动引起的激光强度和干涉信号相位波动也会使得两个波长对应光程差的测量不确定度提高，从而影响距离修正精度。
发明内容
为了克服上述背景技术的不足，本发明提供了一种大范围绝对距离测量中空气折射率的修正方法，采用基于激光单波长与合成波长的双色法实现绝对距离测量中空气折射率的自我修正，为大范围绝对距离测量提供了一种空气折射率的精确补偿方法。
达到上述发明目的，本发明采用的技术方案的步骤如下：
(1)第一激光器输出单波长λ1固定的激光光束，第二激光器依次输出波长从小到大即λ21<λ22<…<λ2i<…<λ2n可调的激光光束，两激光光束经第一偏振分光镜合光后形成正交线偏振光，进入由分光镜、第二偏振分光镜、第一角锥棱镜和第二角锥棱镜构成的激光干涉***；
(2)单波长λ1和可调波长λ21<λ22<…<λ2i<…<λ2n经激光干涉***构建从大到小的合成波长链λS1>λS2>…>λSi>…>λSn，其中λSi＝λ1λ2i/(λ2i-λ1)，i＝1，2，3，…，n，依次对待测的绝对距离D进行测量，得到测量精度逐渐提高的光学距离估计值LS1，LS2，…，LSi，…，LSn，且每级合成波长之间应满足级间过渡条件u(LS(i-1))<λSi/4，其中u(LS(i-1))为通过第i-1级合成波长λS(i-1)测得的光学距离估计值的不确定度i＝2，3，…，n；
(3)首级合成波长λS1的一半要大于待测的绝对距离D对应的光学距离L，即应满足λS1/2>L，最小合成波长λSn要能级间过渡至单波长λ1，即要求u(LSn)<λ1/4；
(4)最小合成波长λSn和单波长λ1对应的光学距离LSn和L1同时测得，根据双色法空气折射率修正原理，得到修正后的绝对距离：
D＝LSn-AS1(LSn-L1)
其中AS1＝(nSn-1)/(nSn-n1)，AS1为λSn和λ1对应的色散系数；nSn和n1分别为λSn和λ1对应的空气折射率，通过Edlén公式计算得到。
所述第一激光器和第二激光器输出激光的波长属于同一个光谱波段，最小合成波长λSn是由同一个光谱波段的两个单波长λ1和λ2n构建。
与背景技术相比，本发明具有的有益效果是：
(1)利用基于激光单波长与合成波长的双色法进行大范围绝对距离测量中空气折射率的自我修正，实现了沿光束实际传播路径的全光路空气折射率的修正，且温度、压强等环境因素的变化对修正精度的影响较小；
(2)最小合成波长λSn与单波长λ1之间的波长差较大，对应的色散系数AS1较小，有利于提高双色法空气折射率的修正精度；
(3)用于构建最小合成波长λSn的两个激光波长λ2n和λ1属于同一个光谱波段，它们的波长值接近，有效降低了测量环境中空气扰动引起的激光强度和干涉信号相位波动对距离修正精度带来的影响，避免了使用不同光谱波段的双色法需要用消除色差影响的特殊透镜和特殊分光镜，减小了光学***的复杂性和造价；
(4)最小合成波长λSn和单波长λ1对应的光学距离LSn和L1同时测得，且测距精度高，减小了光学距离测量误差对双色法空气折射率修正精度的影响。
本发明适用的测距范围大、距离修正精度高，属于沿着实际测量光路的空气折射率全路径修正，且对温度、压强等环境参数的测量精度要求低，易于实现。
附图说明
图1是一种大范围绝对距离测量中空气折射率修正方法原理图。
图中：1、第一激光器，2、第二激光器，3、反射镜，4、第一偏振分光镜，5、分光镜，6、第二偏振分光镜，7、第一角锥棱镜，8、第二角锥棱镜，9、第三偏振分光镜，10、第一光电探测器，11、第二光电探测器，12、干涉信号处理模块，13、计算机；光路中，竖直短线表示偏振方向平行于纸面、波长为λ1的光束，黑点表示偏振方向垂直于纸面、波长为λ2的光束。
具体实施方式
下面结合附图和实施例对本发明作进一步说明。
如图1所示，是本发明的实施例对应的原理图，包括光源***I、激光干涉***II和信号处理***III，具体实施过程如下：
光源***I由第一激光器1、第二激光器2、反射镜3和第一分偏振光镜4组成。第一激光器1采用一台可调谐半导体激光器，输出单波长λ1固定且偏振方向平行于纸面的光束；第二激光器2采用另一台可调谐半导体激光器，输出波长λ2可调且偏振方向垂直于纸面的光束，通过改变其压电陶瓷的驱动电压和 工作电流来调节波长λ2的值；第一激光器1输出的光束直接射向第一偏振分光镜4，第二激光器2输出的光束经反射镜3反射后射向第一偏振分光镜4，二者分别经第一偏振分光镜4透射和反射后合成一束正交线偏振光。
激光干涉***II由分光镜5、第二偏振分光镜6、第一角锥棱镜7和第二角锥棱镜8组成。来自光源***I的正交线偏振光射向分光镜5后，反射光和透射光分别作为参考光束和测量光束。参考光束射向第二偏振分光镜6后，其中，波长为λ2的光束被第二偏振分光镜6反射并射向分光镜5；波长为λ1的光束透过第二偏振分光镜6、经第一角锥棱镜7反射后返回，再次透过第二偏振分光镜6后射向分光镜5。测量光束经第二角锥棱镜8反射后返回至分光镜5，经分光镜5反射后，与透过分光镜5的参考光束合光并发生干涉。
信号处理***III由第三偏振分光镜9、第一光电探测器10、第二光电探测器11、干涉信号处理模块12和计算机13组成。来自激光干涉***II的干涉光束射向第三偏振分光镜9，其中，波长为λ1的干涉光束透过第三偏振分光镜9后，由第一光电探测器10接收，波长为λ2的干涉光束经第三偏振分光镜9反射后，由第二光电探测器11接收，两光电探测器输出的干涉信号经干涉信号处理模块12处理后传输到计算机13，由计算机13计算得到双色法空气折射率修正距离。
具体实施的激光干涉***可采用申请号为201510100565.1的、申请名称为激光干涉波长杠杆式绝对距离测量装置与方法、公开号为104655025A的中国专利申请公开文件中相类似的测量装置。
具体实施中的第一激光器1采用美国Newport公司的一台TLB 6704型可调谐半导体激光器，输出固定的单波长λ1为631nm，第二激光器2采用德国Toptica公司的一台DL Pro 633型可调谐半导体激光器，输出波长λ2的调节范围为630nm－637nm，第一光电探测器10和第二光电探测器11采用二只S09105型PIN光电探测器，干涉信号处理模块12采用德国Struck公司的SIS3316型高速数据采集卡，计算机13采用惠普公司的Pro4500型台式计算机。
结合附图1本发明一种大范围绝对距离测量中空气折射率修正方法的具体实施步骤为：
1.图1中，对于第二激光器2输出波长λ2而言，虚竖线位置至分光镜5的光程与第二偏振分光镜6至分光镜5的光程相等，记此虚竖线位置为测量起点；待测的绝对距离D为测量起点与第二角锥棱镜8之间的几何距离；
2.第一激光器1输出单波长λ1固定的线偏振光；第二激光器2输出波长λ2可调的线偏振光，调节第二激光器2的压电陶瓷驱动电压和工作电流，首先输 出的波长为λ21，且使波长λ21和波长λ1形成的首级合成波长λS1的一半大于待测的绝对距离D对应的光学距离L；
3.把第二角锥棱镜8放在测量起点，调整第一角锥棱镜7，使第一光电探测器10和第二光电探测器11接收到的波长λ1和λ21对应的干涉信号相位同时过零，此时二者之间的相位差
Figure PCTCN2016082279-appb-000001
记此时第一角锥棱镜7的位置为参考起点；
4. 把第二角锥棱镜8放在测量终点(待测的绝对距离D处)，此时第一光电探测器10和第二探测器11接收到的两路干涉信号相位差变为
Figure PCTCN2016082279-appb-000002
再次向远离第二偏振分光镜6的方向移动第一角锥棱镜7，使两路干涉信号的相位再次同时过零，此时有
Figure PCTCN2016082279-appb-000003
记录第一角锥棱镜7的移动位移为Δd1，并设其对应的光学距离为Δl1；移动第一角锥棱镜7返回至参考起点；那么首级合成波长λS1、单波长λ1、待测的绝对距离D对应的光学距离L和Δl1存在如下波长杠杆关系：
Figure PCTCN2016082279-appb-000004
其中λS1＝λ1λ21/(λ21-λ1)，λ1<λ21；
由公式(1)得出通过首级合成波长λS1测量得到的绝对距离对应的光学距离的估计值LS1为：
Figure PCTCN2016082279-appb-000005
5.调节第二激光器2的压电陶瓷驱动电压和工作电流，依次输出波长从小到大分别为λ21<λ22<λ23<…<λ2n，使波长λ2与波长λ1形成逐级递减的合成波长链λS1>λS2>λS3>…>λSn，且每级合成波长之间满足级间过渡条件u(LS(i-1))<λSi/4，其中，i＝2，3，…，n，u(LS(i-1))为第i-1级合成波长λS(i-1)对应的光学距离的估计值LS(i-1)的测量不确定度；由于待测的绝对距离会引起波长λ2i和λ1的两路干涉信号相位差发生变化，通过如步骤4所述的移动第一角锥棱镜7的方法使两路干涉信号的相位同时过零，得第i级合成波长λSi、单波长λ1、第一角锥棱镜7移动位移Δdi对应的光学距离Δli、λSi的小数干涉条纹对应的光学距离ΔLSi之间存在如下波长杠杆关系：
Figure PCTCN2016082279-appb-000006
其中λSi＝λ1λ2i/(λ2i-λ1)；
由于相位差的过零检测是在一个相位周期内进行的，故Δli<λ1/2，ΔLSi<λSi/2，合成波长λSi的干涉条纹小数为：
Figure PCTCN2016082279-appb-000007
由于每级合成波长之间满足级间过渡条件，λSi的整数干涉条纹数MSi由计算机13根据下式算出：
Figure PCTCN2016082279-appb-000008
其中int[]表示向下取整；
合成波长λSi对应的光学距离估计值LSi由计算机13根据下式算出：
Figure PCTCN2016082279-appb-000009
当最小合成波长λSn与单波长λ1之间满足级间过渡条件u(LSn)<λ1/4时，停止调节第二激光器2；
6.在步骤5的最后一次测量，同时得到了λSn与λ1对应的光学距离估计值LSn和L1，根据公式(6)，最小合成波长λSn和单波长λ1对应的光学距离分别为：
Figure PCTCN2016082279-appb-000010
Figure PCTCN2016082279-appb-000011
其中，M1表示单波长λ1的整数干涉条纹数，ε1表示单波长λ1的干涉条纹小数。
由波长杠杆关系：
Figure PCTCN2016082279-appb-000012
其中，ΔLSi表示最小合成波长λSn的小数干涉条纹对应的光学距离，Δln表示第一角锥棱镜7移动位移Δdn对应的光学距离。
得λ1与λSn对应的干涉条纹小数相等，即：
Figure PCTCN2016082279-appb-000013
其中，ε1表示单波长λ1的干涉条纹小数，εSn表示最小合成波长λSn的干涉条纹小数。
7.将采用最小合成波长λSn和单波长λ1测量得到的光学距离LSn和L1直接用于双色法空气折射率的修正，得到修正后的绝对距离：
D＝LSn-AS1(LSn-L1)                   (11)
其中AS1＝(nSn-1)/(nSn-n1)为λSn和λ1对应的色散系数，nSn为λSn对应的群折射率：
Figure PCTCN2016082279-appb-000014
其中n1、n2分别为λ1、λ2n对应的空气相折射率，通过Edlén公式计算得到；
修正后的绝对距离D由计算机13根据公式(11)计算得到。
将公式(7)和公式(8)带入公式(11)，得双色法修正绝对距离的表达式为：
Figure PCTCN2016082279-appb-000015
由于在工业标准环境附近(20℃，101.325kPa，水蒸气分压变化为0.36kPa)，色散系数AS1的变化为0.03，先将AS1视为恒定的常数，分析由λSn的不确定度u(λSn)、λ1的不确定度u(λ1)、λSn小数条纹级次的测量不确定度u(εSn)和λ1小数条纹级次的测量不确定度u(ε1)引起的修正后的绝对距离D的测量不确定度u1(D)，即：
Figure PCTCN2016082279-appb-000016
本实施例中第一激光器1和第二激光器2的输出波长λ1、λ2是经过飞秒光频梳锁定稳频的，它们的相对不确定度为u(λ1)/λ1＝u(λ2)/λ2＝1.0×10-11，由它们所构成的合成波长λSn的相对不确定度为u(λSn)/λSn＝1.0×10-10，λ1的标准不确定度为u(λ1)＝6.31×10-12μm；第一角锥棱镜7的定位精度即Δl的不确定度为u(Δl)＝10-4μm，且Δl<λ1/2；由λ1＝631nm，λ2n＝636nm，得λSn＝80.2μm，AS1＝35.6。标准工业环境下(20℃，101.325kPa，相对湿度50％)，根据Edlén公式得n1＝1.000271391，n2＝1.000271304，根据公式(12)得nSn＝1.000263561；若待测的绝对距离D＝100m，第一角锥棱镜7的移动距离Δl＝0.25μm，将这些参数代入公式(14)得修正后的绝对距离D的测量不确定度为u1(D)＝0.56μm，相对不确定度为u1(D)/D＝5.6×10-9。
当湿度测量不确定度优于±1.0％RH(+15℃～+25℃)和温度测量不确定度优于50mK时，通过分析此双色法空气折射率补偿中由于环境参数的影响引起的修正后的绝对距离D的测量不确定度，得u2(D)/D≈2.4×10-8 。
综合考虑u1(D)和u2(D)得在待测的绝对距离D＝100m时，本发明采用基于激光单波长与合成波长的双色法实现空气折射率自我修正的绝对距离测量相对不确定度为：
Figure PCTCN2016082279-appb-000017
由此可见，本发明提供的一种大范围绝对距离测量中空气折射率的修正方法，在100m测量范围内绝对距离测量的相对不确定度达2.5×10-8。此方法可降低测量环境中空气扰动带来的影响，在绝对距离测量的同时，实现了空气折射率的全路径补偿，修正精度高、应用范围广、易于实现。
上述具体实施方式用来解释说明本发明，而不是对本发明进行限制，在本发明的精神和权利要求的保护范围内，对本发明作出的任何修改和改变，都落入本发明的保护范围。

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  1. 一种大范围绝对距离测量中空气折射率的修正方法，其特征在于，该方法的步骤如下：
    (1)第一激光器输出单波长λ1固定的激光光束，第二激光器依次输出波长从小到大即λ21<λ22<…<λ2i<…<λ2n可调的激光光束，两激光光束经第一偏振分光镜合光后形成正交线偏振光，进入由分光镜、第二偏振分光镜、第一角锥棱镜和第二角锥棱镜构成的激光干涉***；
    (2)单波长λ1和可调波长λ21<λ22<…<λ2i<…<λ2n经激光干涉***构建从大到小的合成波长链λS1>λS2>…>λSi>…>λSn，其中λSi＝λ1λ2i/(λ2i-λ1)，i＝1，2，3，…，n，依次对待测的绝对距离D进行测量，得到测量精度逐渐提高的光学距离估计值LS1，LS2，…，LSi，…，LSn，且每级合成波长之间应满足级间过渡条件u(LS(i-1))<λSi/4，其中u(LS(i-1))为通过第i-1级合成波长λS(i-1)测得的光学距离估计值的不确定度i＝2，3，…，n；
    (3)首级合成波长λS1的一半要大于待测的绝对距离D对应的光学距离L，即应满足λS1/2>L，最小合成波长λSn要能级间过渡至单波长λ1，即要求u(LSn)<λ1/4；
    (4)最小合成波长λSn和单波长λ1对应的光学距离LSn和L1同时测得，根据双色法空气折射率修正原理，得到修正后的绝对距离：
    D＝LSn-AS1(LSn-L1)
    其中AS1＝(nSn-1)/(nSn-n1)，AS1为λSn和λ1对应的色散系数；nSn和n1分别为λSn和λ1对应的空气折射率，通过Edlén公式计算得到。
  2. 根据权利要求1所述的一种大范围绝对距离测量中空气折射率的修正方法，其特征在于：所述第一激光器和第二激光器输出激光的波长属于同一个光谱波段，最小合成波长λSn是由同一个光谱波段的两个单波长λ1和λ2n构建。
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