CN201897503U - A Wide Spectrum Spatial Heterodyne Spectrometer - Google Patents

Abstract

The utility model discloses a wide spectral range space heterodyne spectrum appearance has overcome the very narrow defect of traditional space heterodyne spectrum appearance spectral range among the prior art. The blazed grating component in the utility model consists of a first echelle grating on a reflection light path and a second echelle grating on a transmission light path which are respectively formed by beam splitting elements for the first time; the angle and position relation of the two medium echelle gratings meets the following requirements: (1) both the reflected light beam and the transmitted light beam which are split by the beam splitting element for the first time are incident on the two echelle gratings at a Littrow angle, and (2) the first echelle grating rotates for alpha/4 degrees around the first rotating shaft and the second echelle grating rotates for negative alpha/4 degrees around the second rotating shaft by taking the position relation of the conventional blazed grating component and the beam splitting element as a reference. The utility model has the advantages of high stability, high spectral resolution, wide spectral band cover, the application that especially adapted spaceborne space environment remote sensing and atmosphere were surveyed.

Description

A Wide Spectrum Spatial Heterodyne Spectrometer

technical field

The utility model relates to a wide-band spatial heterodyne spectrometer.

Background technique

1. Mathematical expression of the basic principle of spatial heterodyne spectrometer

The radiation passing through the entrance pupil of the interferometer becomes a plane wave after being collimated, split by the beam splitter and incident on two gratings at the Littrow angle θ _L . According to the grating equation

σ(sinθ _L +sinβ) = mD, (1)

σ is the wave number, θ _L is the beam incident angle, β is the beam exit angle, m is the diffraction order, and D is the groove density of the grating. Then the light with different wavenumber σ emerges from the grating at different diffraction angles β, and there will be light with a certain wavenumber σ ₀ in the diffracted outgoing beam according to the original incident light path, that is, corresponding to β(σ ₀ )=θ _L , corresponding to The outgoing wave surfaces of the two gratings are parallel to each other after passing through the beam splitter, and this wave number is called the heterodyne wave number. The outgoing wavefronts corresponding to other wave numbers σ will have a certain angle 2γ, and the size of the γ angle can be solved from the β angle. In this way, for an incident wave surface, there are two outgoing wave surfaces corresponding to it, and these two wave surfaces will interfere, satisfying the formula:

I(x)=∫B(σ)(1+cos(2π(4(σ-σ ₀ )xtanθ _L )), (2)

The interferogram can be obtained along the detected x direction, and the spectrum B(σ) of the incident light can be obtained by further Fourier transforming the interferogram I(x).

2. Existing Research Situation

At present, Ye Song and others from the Anhui Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences have carried out research on spatial heterodyne spectrometers in China. It mainly studies the traditional narrow-band spatial heterodyne spectrometer using ordinary blazed gratings as dispersive elements, as shown in Figure 1.

The blaze angle of ordinary blazed gratings is about 20°~30°; the density of the grooves is generally several hundred to one or two thousand gr/mm; it is generally used for first-order diffraction, that is, m=1. Spatial heterodyne spectroscopy is only established in a small angle range close to the blaze angle θ _L , and only one order is used with an ordinary grating, so the spectral band will be very narrow.

Relevant foreign studies have proposed the feasibility of using echelle gratings, because the blaze angle of echelle gratings is large, generally 60°-70°; the density of reticles is small, generally tens of gr/mm; the use order is high, generally Dozens to hundreds of orders, that is, m=n×10~100, and dozens of diffraction orders are used at the same time. However, how to combine a series of narrow spectral bands into a wide spectral band by using many orders at the same time, especially the corresponding design of the optical structure and the processing of the subsequent optical path, is only in the early stage of theoretical research and is not yet mature.

Although there is no order aliasing problem with ordinary gratings, it is still necessary to solve the problem of restoring spectral aliasing (the spectra of σ ₀ ±Δσ will be mixed together). Two approaches have been adopted in existing studies: first, a band-pass filter wheel is added at the front of the entrance pupil, so that the light with σ> _σ0 and σ< _σ0 enters the interferometer successively and is collected separately. Second, also tilt the two gratings separately (the tilt angle is not strictly limited). For instruments used in spaceborne remote sensing, their stability and reliability are very important indicators. It is not desirable to have moving parts in the system, otherwise this will bring risks to stability and reliability. Components such as mechanical mask wheels are also equipped with drive motors and precision control systems, which will put pressure on the size, weight and power consumption of the instrument.

Utility model content

The utility model provides a wide-band spatial heterodyne spectrometer, which overcomes the defect that the traditional spatial heterodyne spectrometer in the prior art has a very narrow spectrum.

The technical scheme of the utility model is as follows:

A wide-spectrum spatial heterodyne spectrometer, including an interferometer entrance pupil, a collimation system, a beam splitting element, a blazed grating assembly, a fringe imaging system, and a detector array, wherein the core component of the beam splitting element is a 50:50 semi-anti-half The difference between the transparent depolarizing spectroscopic film and the existing narrow-band spatial heterodyne spectrometer (described in the background technology) is that the blazed grating assembly is composed of the first echelle grating on the reflection optical path formed by the beam splitting element for the first time and the second echelle grating on the transmitted light path; let the line passing through the center of the first echelle grating and parallel to the optical axis of the optical path where the collimation system is located be the first rotation axis, pass through the center of the second echelle grating and be on the same plane The straight line perpendicular to the first rotation axis is the second rotation axis, and it is stipulated that the incident direction of the echelle grating and then rotated 90 degrees counterclockwise is the positive direction of the rotation axis, and the angle and position relationship of the two echelle gratings satisfy : (1) The reflected and transmitted beams split by the beam splitting element for the first time are both incident on the two echelle gratings at the Littrow angle, (2) Based on the positional relationship between the conventional blazed grating assembly and the beam splitting element, the first echelle The grating rotates α/4 degrees around the first rotation axis, and the second echelle grating rotates negative α/4 degrees around the second rotation axis, where α≥4sinθ _L /(DW _y ), W _y is the grating along the reticle direction The size of D is the grating line density; the fringe imaging system includes a front mirror group and a rear mirror group, the rear focal plane of the front mirror group coincides with the front focal plane of the rear mirror group, and the overlapping position of the focal plane is provided with an order Choose a mask.

It is better to choose a controllable optical shutter for the above-mentioned order selection mask, so that the light transmission and light blocking states of each area can be completely controlled by circuit programming, with small size, low power consumption, and faster response speed than mechanical masks. There are no moving parts.

The best form of the above-mentioned controllable optical shutter is that there are four quadrant independent control partitions, which can realize the diffraction order selection function by synchronously controlling the light transmission or light blocking states of two groups of diagonal quadrant regions.

The above-mentioned fringe imaging system is preferably a 4f system, and the order selection mask is located at the position of the spectrum plane of the 4f system.

Specific forms such as a beam splitting plate or a beam splitting prism can be selected for the above beam splitting element.

The above-mentioned collimation system can be a transmissive system or a reflective system.

The telescopic system in front of the entrance pupil of the above-mentioned interferometer can be a transmissive system or a reflective system.

The utility model has the following advantages:

1. The utility model borrows the existing equipment and optical path structure to realize a wide-spectrum spatial heterodyne spectrometer, which overcomes the defect that the traditional spatial heterodyne spectrometer in the prior art has a narrow spectrum.

2. The four-quadrant optical shutter mask completely controls the light-transmitting and light-blocking states of each area through circuit programming, with small size, low power consumption, and faster response speed than mechanical masks. The most important thing is that there is no need for driving motors and precise control circuits like mechanical masks. As long as the mask installation accuracy is ensured, the entire working process does not require position adjustment. The system also does not require bandpass filters like narrowband systems. Such a static wide-spectrum spatial heterodyne spectrometer has no moving parts, and the system has the advantages of high stability, high spectral resolution, and wide-spectrum coverage. It is very suitable for spaceborne space environment remote sensing and atmospheric detection applications.

Description of drawings

Fig. 1 is the principle schematic diagram of the spatial heterodyne spectrometer of prior art narrow band;

Fig. 2 is a schematic diagram of the principle of the utility model wide-band space heterodyne spectrometer (when the _x1 axis and the _x2 axis are not rotated);

Figure 3 is a schematic diagram of the diffraction order on the mask surface;

Figure 4 shows two working states of the mechanical mask;

Fig. 5 is two working states of the four-quadrant optical shutter in the embodiment of the present utility model; Wherein, the state shown in figure a is the AC light transmission of the four-quadrant optical shutter, BD light-blocking, the state shown in figure b is the four-quadrant optical The AC of the shutter is light-blocking, and the BD is light-transmitting.

Explanation of reference numbers:

1-interferometer entrance pupil, 2-collimation system, 3-blazed grating, 31-echelle grating, 4-beam splitting element, 5-fringe imaging system, 6-detector, 7-order selection mask.

Detailed ways

The basic principle of the utility model is: the telescopic system collects the spectral radiation of the observation target, and images it to the entrance pupil of the interferometer, and the target radiation passing through the entrance pupil of the interferometer becomes a parallel beam after being passed through the collimation system, and is captured by the beam splitter It is divided into transmission and reflection, which are respectively incident on two echelle gratings at the Littrow angle (θ _L ). After being dispersed by the echelle grating, beams of different wavelengths are reflected from the grating back to the beam splitter at different diffraction angles. Then through the beam splitter, it is semi-reflective and semi-transparent, and is merged into an outgoing beam at the exit of the interferometer. In this way, for an incident wavefront, there are two outgoing wavefronts with a certain angle between them, and there is coherence between the two wavefronts. The fringe imaging system L ₁ converges the outgoing beams, and obtains images of the entrance pupil of the interferometer corresponding to different wavelengths and diffraction orders produced by the dispersion of the two stepped gratings on the mask surface. The mask transmits the desired diffraction orders and blocks unwanted ones. The fringe imaging system L ₂ then converts the beam passing through the mask into a parallel beam, and the beams from the two echelle gratings interfere on the rear focal plane of L ₂ , and the interference pattern is received by the detector array. Perform two-dimensional Fourier transform and data restoration processing on the interferogram to obtain the spectrum of the target radiation.

In this embodiment, a four-quadrant controllable optical shutter is selected as a diffraction order selection mask, thus forming a static wide-band spatial heterodyne spectrometer. It consists of interferometer entrance pupil, collimation system, beam splitter, echelle grating, fringe imaging system, order selection mask and detector.

1) Interferometer entrance pupil: located on the rear focal plane of the telephoto system and the front focal plane of the collimation system, the function is to control the amount of incident energy and the angle of view of the interferometer.

2) Collimation system: convert the beam passing through the entrance pupil of the interferometer into a parallel beam. It can be a transmissive system or a reflective system.

3) Beam splitter: Its core is a 50:50 semi-reflective and semi-transparent depolarized spectroscopic film, which divides the incident parallel beam into two beams of transmission and reflection, and the polarization characteristics of each beam of parallel light are the same as those of the incident parallel light. same. The specific structure can be a beam splitting plate or a beam splitting prism.

4) Echelle grating: a reflective grating with a large blaze angle, relying on high diffraction orders to achieve high spectral resolution. In this system, two echelle gratings are symmetrically placed relative to the beam splitter, so that the two beams of parallel light emitted by the beam splitter are incident on the two echelle gratings at the Littrow angle (θ _L ), and the direction of the grating lines is approximately perpendicular to On the paper, the two gratings are respectively rotated by an angle of α/4 around the x ₁ axis inside the paper and outwards around the x ₂ axis inside the paper. After being dispersed by the grating, beams of different wavelengths emerge from the grating at different diffraction angles β. Beams of the same wavelength are diffracted by the two gratings at the same diffraction angle, but there will be an included angle of α along the direction perpendicular to the paper.

5) Fringe imaging system L ₁ : the parallel light beams with different diffraction angles reflected from the two echelle gratings are converged onto the mask of the rear focal plane.

6) Mask: Select the diffraction order required for interference. The position is on the back focal plane of _L1 and the front focal plane of _L2 .

7) Fringe imaging system L ₂ : transform the beam passing through the mask into a parallel beam, and form a Fizeau-style interferogram on its rear focal plane.

8) Detector array: located on the rear focal plane of the fringe imaging system _L2 , the function is to receive the interferogram.

Among them, the four-quadrant controllable optical shutter is a spatial light modulation device, and its light transmission and light blocking states can be controlled by electrical signals. The four-quadrant controllable optical shutter has four independently controlled partitions. By synchronously controlling the light transmission and light blocking states of the AC area and BD area, the diffraction order selection function can be realized. Compared with mechanical masks, the four-quadrant controllable optical shutter has no moving parts, and is small in size and light in weight. The whole instrument has no moving parts, good stability and high reliability, and is suitable for aerospace remote sensing applications.

The optical path structure of the static wide-band spatial heterodyne spectrometer is:

(1) The incident aperture of the interferometer is located on the front focal plane of the collimation system; (2) The on-axis beam is incident on the beam splitting film of the beam splitting element at an angle of 45°, forming two paths of transmitted and reflected light. The optical axis of the transmitted beam coincides with the optical axis of the collimating mirror, and the optical axis of the reflected beam is perpendicular to the optical axis of the collimating mirror. (3) The positional relationship of the two echelle gratings satisfies: first, the grating lines are perpendicular to the plane determined by the optical axis of the transmitted beam and the optical axis of the reflected beam, that is, the main section of the grating is in this plane, and the reflected and transmitted beams are both in the plane The Littrow angle is incident on the two échelle gratings. Then, based on the positional relationship between the conventional (in the background art) blazed grating assembly and the light splitting element, the first echelle grating is rotated by α/4 degrees around the first rotation axis (i.e. the _x1 axis), and the second echelle The grating is then rotated negative α/4 degrees around the second rotation axis (ie, the _x2 axis); (4) The optical axis of the fringe imaging system is perpendicular to the optical axis of the collimation system, and coincides with the optical axis of the original (first) reflected beam. The fringe imaging system can be divided into a front mirror group and a rear mirror group. The rear focal plane of the front mirror group coincides with the front focal plane of the rear mirror group, and an order selection mask is set at the overlapping position of the focal planes. The area array detector is located on the rear focal plane of the rear mirror group.

Spatial heterodyne spectroscopy is only established in a small angle range close to the blaze angle θ _L , and only one order is used with an ordinary grating, so the spectral band will be very narrow. Using an ordinary blazed grating as a dispersive element does not encounter the problem of overlapping diffraction orders, so there is no term containing α angle in formula (2). The use of echelle gratings needs to solve the problem of order overlap, so the two gratings are respectively tilted by α/4, and many orders are used. Each order has a Littrow wavenumber σ _0m , which can be composed of a series of narrow spectral segments Synthesize broad bands. Thus the interference formula becomes

I(x)＝∫B(σ)(1+cos(2π(4(σ-σ _0m )xtanθ _L +αyσ)) (3)

Generally speaking, there are two considerations for the value of α. First, it depends on the characteristic parameters of the echelle grating. Second, according to the spectral range, fringe imaging system and detector parameters used in the instrument design. Specifically, the inclination angle α/4 must be large enough to make a series of Littrow wavenumbers σ _0m separate at least one interference fringe in the y direction, which should satisfy α≥4sinθ _L /(DW _y ), D is the grating line density, W _y is the dimension of the grating along the reticle direction. The _upper limit of α should be determined according _to the specific spectral range, _detector image size and fringe imaging system parameters _. The pixel size of the device. At present, when designing the system, α=M/(2σ _max d _pix ) is usually taken, where M is the magnification of the fringe imaging system.

Claims (7)

1. a wide spectrum space heterodyne spectrograph comprises interferometer entrance pupil, colimated light system, beam splitting element, blazed grating assembly, striped imaging system and detector array, and wherein the core component of beam splitting element is the depolarization spectro-film of 50: 50 half-reflection and half-transmissions.It is characterized in that: described blazed grating assembly is formed by being in first echelle grating on the reflected light path that beam splitting element forms first and second echelle grating on the transmitted light path respectively; If through the first echelle grating center and the straight line parallel with the optical axis of colimated light system place light path is first turning axle, through the second echelle grating center and in same plane the straight line vertical with described first turning axle be second turning axle, and all the incident direction of regulation echelle grating is rotated counterclockwise 90 degree again and is the positive dirction of rotation, the angle of two echelle grating, the position relation satisfies: reflection that (1) beam splitting element is told first and transmitted light beam all incide on two echelle grating with the Littrow angle, (2) be benchmark with the blazed grating assembly of routine and the pass, position of beam splitter, first echelle grating is spent rotation alpha/4 around first turning axle again, second echelle grating is rotated negative α/4 degree, wherein α 〉=4sin θ again around second turning axle _L/ (DW _y), W _yBe the size along groove direction grating, D is a grating line density; Mirror group and back mirror group before described striped imaging system comprises, the back focal plane of preceding mirror group overlaps with the front focal plane of back mirror group, and this focal plane overlaps position and is provided with the inferior mask of selecting of level.

2. wide spectrum space heterodyne spectrograph according to claim 1 is characterized in that: described level time selection mask is the controllable optical shutter.

3. wide spectrum space heterodyne spectrograph according to claim 2, it is characterized in that: described controllable optical shutter exists four-quadrant independently to control subregion, can realize the order of diffraction time selection function by the printing opacity or the resistance light state in two groups of diagonal quadrant zones of synchro control.

4. according to the arbitrary described wide spectrum space heterodyne spectrograph of claim 1 to 3, it is characterized in that: described striped imaging system is the 4f system, and described level time selection mask is positioned at the frequency plane position of this 4f system.

5. wide spectrum space heterodyne spectrograph according to claim 4 is characterized in that: described beam splitting element is beam-splitting board or beam splitter prism.

6. wide spectrum space heterodyne spectrograph according to claim 4 is characterized in that: described colimated light system is transmission-type system or reflect system.

7. wide spectrum space heterodyne spectrograph according to claim 4 is characterized in that: the telescopic system in described interferometer entrance pupil the place ahead is transmission-type system or reflect system.

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