Interferometry: using multiple rackstelescopeCombine light or radio waves from the same celestial body to increase decomposition.The common ones areOptical interferometryLength measurement, interference spectrum, radio interference measurement.
Interferometry of visible light isInterferometryIn the early practical applications, such as the Michelson star interferometer for measuring the angular diameter of stars, how to obtain a stable coherent light source is always one of the important reasons that limit the development of optical measurement.Until the 1960s,Optical interferometryTechnology has developed rapidly, thanks to the invention of laser, a high-intensity coherent light source, computers, etcDigital integrated circuitThe ability to acquire and process the data obtained by the interferometer has been greatly improved, and the application of single-mode fiber has increased the effective optical path in the experiment and still can maintain very low noise [25].With the development of electronic technology, it is no longer necessary to observe the interference fringes produced by interferometers, but can directly measure the phase difference of coherent light.Some important applications of optical interferometry in many aspects are listed here.
Length measurement
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Used to measure the change of optical path difference, and then determine the refractive index of gasRayleigh interferometer
Length measurement isOptical interferometryOne of the most common applications.To measure the absolute length of a sample, one of the most concise methods is to count the interference fringes generated by interference;If non integer interference fringes are encountered, narrower interference fringes can be obtained by continuously doubling the wavelength of coherent light until satisfactory measurement accuracy is obtained.Common methods also includeHPThe Hewlett Packard interferometer developed by us makes the He Ne laser work at two similar frequencies by adding an axial magnetic field, thus emitting two lasers with a frequency difference of 2 MHz, and then makes the two lasers produce heterodyne interference through a polarization beam splitter.The differential frequency signal obtained from the interference is recorded by the optical detector, and the change of optical path difference caused by the sample to be measured can be expressed as an integral multiple of the optical wavelength through the counter.HP interferometer can measure the length within 60 meters, and can also be used to measure angle, thickness, flatness and other occasions after adding other optical devices.In addition, the difference frequency signal can also be obtained through the acousto-optic modulation method, and this method can obtain a higher difference frequency, so that a higher count can be obtained from the difference frequency signal.
Another type of length measurement is to measure the change of length. Common methods such as heterodyne interference generated by acousto-optic modulation, the phase difference carried by the differential frequency signal will be recorded by the optical detector to obtain the change of length.When measuring, like fused quartzCoefficient of thermal expansionWhen the thermal expansion coefficient of the material is very low, a more accurate method is often used: place the glass plate with transmission and reflection on both sides at both ends of the sample to be measured, thus forming aFabry Perot interferometer。Two heterodyne interference lasers are used, and the frequency of one of them is locked to a transmission peak frequency of the Fabry Perot interferometer through feedback.In this way, when the thermal expansion of the sample changes the length of the Fabry Perot interferometer, the change of the transmission peak frequency will cause the corresponding change of the locked laser frequency, which will also be reflected in the heterodyne signal and detected.
Interference spectrum
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useSOHO satelliteThe solar corona observed by the LASCO C1 camera.The 5308Å of iron XIV was measured accurately by Fabry Perot interferometer;Multiple wavelengths of the spectral line. These wavelengths generate Doppler frequency shift due to the relative motion of the plasma in the corona and the detection satellite. Different colors are used to represent different degrees of Doppler frequency shift photos, so different colors also represent different relative velocities.
The ratio of the central wavelength of the two spectral lines that can be resolved by the spectrometer to the wavelength difference that can be just resolved is called the color resolution of the spectrometer.For those using dispersion effectPrism spectrometerAnd using the diffraction effectGrating spectrometerIts color resolution ability will not exceed the order of 106.However, ifFabry Perot interferometer, because the half width of the transmission peak is equal to the free spectral range of the interferometer divided by its fineness:
And by substituting the interference condition 2nd=m lambda , we can get
\Delta nu= frac { nu} {m mathcal {F}} , where nu is the center frequency.
thusFabry Perot interferometerThe color resolving power of is frac { nu} { Delta nu}=m mathcal {F} ,.Generally, the interference order is m sim 10 ^ 5 , and the fineness is mathcal {F} , at least 10 sim 10 ^ 2 , so the color resolution of the interference spectrometer is above the order of 106 to 107.
Another important application of interferometers is the manufacture of wavelength meters, which are divided into dynamic wavelength meters and static wavelength metersMichelson interferometerOr the free spectrum range is multipleFabry Perot interferometerCombined.In addition, the heterodyne interference of the laser, combined with the Fabry Perot interferometer, can be used to measure the frequency of the laser more accurately or to compare the frequency of two lasers, and the linewidth of the laser can also be measured through acousto-optic modulation and fiber delay.
Radio interferometry
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The angular resolution of the telescope is proportional to the wavelength divided by the aperture. Since the wavelength of the radio wave is far longer than the visible light, this results in a singleradio telescopeThe resolution required for observing ordinary radio sources cannot be reached (for example, the radio wave with a wavelength of 2.8 cm is used for observation with a resolution of 1 millisecond, which requires a telescope aperture of 6000 km).For this reason, British astronomer Sir Martin Ryle and others invented the radio interference technology in 1946. They used a two antennaRadio interferometerThe sun was observed.Radio interference technology uses multiple discrete radio telescopes to form an array. These telescopes are aligned with the same radio emission source when observing. The signals observed by each other are connected with coaxial cables, waveguides or optical fibers before interference.This interference not only improves the strength of observation signals, but also improves the effective aperture of observation due to the long baseline distance between telescopes.Due to the different positions of each telescope, the time when the same wave front reaches each telescope will be delayed, so it is necessary to properly delay the signals that arrive first to maintain the time coherence between the signals.In addition, the more telescopes constitute interference, the better. This is because when observing the light intensity distribution on the radio source surface, the interference composed of two telescopes can only observe the light intensity distributionFourier transformation One of the spatial frequencies (here, spatial frequency means Fourier frequency describing how fast light intensity changes in different directions) of (visibility);By using multiple telescopes to form an array, radio sources can be observed at multiple spatial frequencies, and thenVisibility functionThe inverse Fourier transform is used to obtain the light intensity distribution of the radio source. This method is called synthetic aperture.For example, in New MexicoVery large antenna array(VLA) consists of 27 radio telescopes, each consisting ofParabolic antennaThey form a total of 351 mutually independent interference baselines, and the longest equivalent baseline can reach 36 kilometers.
In the late 1960s, with theradio telescopeWith the improvement of the performance and stability of the receiver, it is possible to generate interference between the same radio signal far away from the telescope in the world (even in Earth orbit), which is called ultra long baseline interference (VLBI).Ultra long baseline interference does not require physical connection between observation signals, but embeds the time information calibrated by atomic clock in the signal data itself, and then calculates the correlation of these data.Since these data are observed at distant locations, the equivalent baseline can be very long.The ultra long baseline interferometers already in operation include the ultra long baseline array (8611 km in length) located in the US mainland and overseas territories, and the European ultra long baseline interferometers network spread across Eurasia and Africa.These interference arrays usually carry out independent observation, but can achieve simultaneous observation in some special projects, thus forming a global ultra long baseline interference.
