Step grating

Physical noun

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Chinese name: Step grating
Foreign name: echelon grating

Nature: grating Dispersive element A form of
Discipline: Physics
Definition: High diffraction order diffraction grating

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brief introduction

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Step grating （ Echelle grating , from French é chelle, ladder) is a kind of reticle with low density, but the shape of reticle is for high incident angle, that is, high diffraction order diffraction grating 。 High diffraction order can further dispersion the spectrum, thus providing more detailed characteristics for the detector. Ladder gratings are used in the same way as other types of diffraction gratings a spectrometer And other similar instruments. Ladder gratings are most commonly used in high-resolution transverse dispersion spectrographs, especially Extrasolar planets On detectors such as High precision radial velocity planet finder （HARPS）、 Indian Physical Research Laboratory Advanced Radial Velocity All day Sky Survey (PARAS) and many other astronomical instruments. Generally, it can be divided into three types: large, medium and small. Generally speaking, the grating with less than 10 lines per mm is called large step grating. 10 to 400 lines per mm, called Middle step grating 。 More than 400 items are called Small step grating 。

Development history

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The earliest concept of grazing angle grating originated in 1898 from Albert Michelson Propose that,^[1] And this type of grating is called "echelon". However, this concept was not realized until 1923, when the ladder of modern form appeared Grating spectrometer ； High resolution gratings are arranged in series in the form of cross low dispersion. This configuration is made by Bantaro Nagaoka They found that the later designs are very similar to those proposed by Nagaoka.

principle

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Like other diffraction gratings, ladder gratings are conceptually composed of many slits whose width is similar to the wavelength of the diffracted light source. The single wavelength ray perpendicular to the standard grating will be diffracted to the central zero order and continuous high-order region at a specific angle. The degree of diffraction depends on the grating density to wavelength ratio and the selected order. The separation angle of each higher-order diffraction decreases monotonically and reaches a very close degree, but the lower order part will be completely separated. The intensity of the diffraction pattern can be changed by changing the tilt angle of the grating. The part of the reflection grating (the grating hole is replaced by a high reflectivity plane) can tilt Scatter most of the light source to the direction required by the user (and the specific diffraction order). It can be realized for multi wavelength light source, but higher order long wavelength diffracted light may cover lower order light with shorter wavelength, which is usually a side effect that users do not want to happen.

However, the design of the step grating is intended to cover the higher-order diffracted light, and the flicker state is optimized to apply to the state where multiple higher-order diffracted light overlap. Because overlapping rays cannot be directly applied, a secondary dispersion element (grating or prism) must be vertically installed on the optical path to separate diffracted rays of different orders or cross separate them according to the order on the beam path. Therefore, the generated spectrum will be a continuous multi band banded oblique pattern projected on a specific imaging plane, including different wavelengths, but with a few areas overlapping. It is this design that can overcome the imaging problem of broadband high-resolution spectral equipment. Therefore, it is often used in extremely long linear sensor arrays or will generate strong Defocus aberration Other types aberration Optical system. Stepped gratings make it possible to manufacture two-dimensional sensor arrays easily, thus reducing measurement time and increasing efficiency.

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diffraction grating
Radial velocity method for exoplanet detection

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