US 2013/0258348 A1Published application

Method and apparatus for measuring the refractive index of air based on the laser synthetic wavelength interferometry

AssigneeZhejiang Sci-Tech University

InventorsLiping Yan, Benyong Chen, Qiuhong Tian, Zhengrong Sun

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Abstract

The present invention discloses a method and an apparatus for measuring the refractive index of air based on the laser synthetic wavelength interferometry, wherein a laser synthetic wavelength interferometer comprises a dual-frequency laser, a beamsplitter, a first polarizing beamsplitter, a second polarizing beamsplitter, a first corner-cube retroreflector and a second corner-cube retroreflector; a quartz vacuum cavity is disposed in the measuring optical path in parallel to the light propagation direction; when the measurement is performed, air is introduced into the quartz vacuum cavity until it is consistent with outside environment; the variation of the refractive index of air inside the cavity will cause change of the interference signal of the wavelength λ2; the first detector of photoelectric type is used to directly detect the integer N of the interference fringes of the wavelength λ2; then the first corner-cube retroreflector is moved to make the phase difference between the interference signals of the wavelengths λ1 and λ2 equal to the initial phase difference before introduction of the air into the quartz vacuum cavity; the fraction ε of the interference fringes of the wavelength λ2 is measured and obtained according to the principle of subdivision of laser synthetic wavelength interference fringes; and the refractive index of air n is obtained by using the length L of the vacuum cavity, the integer N and the fraction ε of the interference fringes of the wavelength λ2. The measurement according to the present invention has the accuracy up to 10−9 or higher, and has a strong anti-disturbance ability to the environment.

A method for measuring the refractive index of air based on the laser synthetic wavelength interferometry, characterized in that, the method comprises the steps of:

using a dual-frequency laser to output orthogonal linear polarization lights with wavelengths λ1 and λ2 ;
emitting the orthogonal linear polarization lights to a laser synthetic wavelength interferometer composed of a beamsplitter, a first polarizing beamsplitter, a second polarizing beamsplitter, a first corner-cube retroreflector and a second corner-cube retroreflector;
in the measuring optical path of the interferometer, a quartz vacuum cavity of a length L being disposed in parallel to the light propagation direction;
receiving interference signals of the wavelengths λ1 and λ2 respectively by a first detector and a second detector when moving the first corner-cube retroreflector of the interferometer;
before starting to measure the refractive index of air, evacuating the quartz vacuum cavity, moving the first corner-cube retroreflector to find a position where the phase difference between the two interference signals of the wavelengths λ1 and λ2 is set to an original value of zero;
then introducing air into the quartz vacuum cavity until the air inside the cavity is consistent with air outside, during this process, taking the interference signal of the wavelength λ1 as a reference signal;
firstly, using the first detector of photoelectric type to directly detect the integer N of the interference fringes of the wavelength λ2;
and then moving the first corner-cube retroreflector such that the phase difference between the interference signals of the wavelengths λ1 and λ2 becomes zero again;
according to the principle of subdivision of laser synthetic wavelength interference fringes, measuring and obtaining the fraction ε of the interference fringes of the wavelength λ2 resulted from variation of the refractive index of air;
and finally, calculating the refractive index of air according to the relation of the refractive index of air n to the length L of the vacuum cavity, the number N and ε of the interference fringes as follows:
n
-
1
=
(
N
+
ɛ
)
·
λ
20
2

L
Wherein λ20 is the vacuum wavelength.

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Abstract

Claims2