CN100427906C - Total reflection type Fourier transform imaging spectrometer employing Fresnel double-mirror - Google Patents

Abstract

The present invention relates to a total reflection type Fourier transformation imaging spectrometer which utilizes a Fresnel double-faced reflector. The present invention is composed of an optical structure, a focal plane detector and a signal collecting and processing system, wherein the optical structure comprises a reflection type front telescope, a slit, a reflection type collimating lens, a Fresnel double-faced reflector, a reflection type cylindrical lens, etc. Incident light is analyzed into two coherent beams having a certain intersection angle by the Fresnel double-faced reflector, a one-dimensional interference fringe is produced at the surface of the focal plane detector, and the other dimensional gray level image is focused to form images by the cylindrical lens. A two-dimensional focal plane detector adopts any band from the visible light to the long-wave infrared (from 0.4 to 14 um). The signal collecting and processing system can obtain images and the spectral distribution of every point on the slit. A target spectral image data cube can be obtained by the conical scan along the direction which is perpendicular to the slit. The present invention has the advantages of wide wave band of the spectral response, large luminous flux, high signal-to-noise ratio and simple structure. The present invention is especially suitable for the remote sensing hyper-spectral imaging field in the aviation and the spaceflight.

Description

Adopt the total reflection type Fourier transform imaging spectrometer of Fresnel double-mirror

Technical field

The present invention relates to a kind of total reflection type Fourier transform imaging spectrometer, belong to the designing technique category of imaging spectrometer in the remote sensing technology field; Be particularly related to the design of Optical System of spatial modulation and interference type Fourier transform imaging spectrometer.

Background technology

Fourier transform imaging spectrometer (Fourier Transform imaging spectrometer) is called interference type imaging spectrometer (imaging interferometer) again.Compare with traditional color dispersion-type imaging spectrometer, Fourier transform imaging spectrometer has the input characteristics that luminous flux is big, spectral resolution is high, therefore is particularly suitable for the high light spectrum image-forming (Hyper spectralimaging) in the Aero-Space remote sensing field.On optical principle, Fourier transform imaging spectrometer can be divided into time modulation (Temporari1y Modulated) inteference imaging spectrometer and spatial modulation (Spatially Modulated) inteference imaging spectrometer two big classes.The former is representative with Michaelson (Michelson) Fourier transform imaging spectrometer that relies on index glass scanning; Typical case's representative of the latter mainly contains the Fourier transform imaging spectrometer that adopts Sagnac beam splitting structure or its variant, and birefringece crystal beam splitting formula is a polarization interference formula Fourier transform imaging spectrometer etc.Document [3], [4], [5] have been introduced the spatial modulation type Fourier transform imaging spectrometer that adopts Sagnac type beam splitting interference part respectively; Document [6], [7] have been introduced the spatial modulation type Fourier transform imaging spectrometer that adopts polarization-type beam splitting interference part respectively.Notice that said apparatus has all adopted Transflective beam-splitting board or transmission-type light splitting part in their interference system.

Patent aspect at home, Chinese patent No.99115952[8] and No.99256131[9] introduced the interference type imaging spectrometer that adopts Sagnac beam splitting structure respectively; No.01213109[10], No.01213108[11] and No.99256129[12] introduced the interference type imaging spectrometer that adopts the polarization-type device respectively.The common feature of foregoing invention device is all to comprise Transflective or transmission-type optics.

Aspect international monopoly, U.S. Pat 4523846[13] and US5777736[14] introduced the interference type imaging spectrometer that adopts Sagnac beam splitting structure respectively, the two all comprises Transflective or transmission-type optics; U.S. Pat 5260767[15] though introduced a kind of total-reflection type imaging spectrometer, what it adopted is the color dispersion-type beam-splitting structure, does not belong to the type of Fourier transform imaging spectrometer.

In sum, Fourier transform imaging spectrometer has up to now all inevitably adopted the beam splitting interference structure of Transflective or transmission-type.On principle, there are a lot of problems in Transflective or transmission-type beam splitting interference structure.One, spectral range is restricted; The base material of Transflective beam-splitting board is selective to optical band; Simultaneously, it is also relevant with wavelength to be plated in the splitting ratio of beam splitting coating of substrate surface; The result causes must adopting in imaging spectrometer a plurality of beam-splitting boards with different spectral characteristics just can finish branch beam interferometer from visible light to infrared band, thereby strengthens the complexity of total system structure.Its two, optical energy loss is bigger, the beam splitting device reflects the asymmetry of ratio thoroughly and all can cause optical energy loss on the surface of beam-splitting board and the inner optical loss that exists, this problem is unfavorable for the high light spectrum image-forming under the low-light (level).Its three, polarization problem, beam splitting coating causes two-beam to have polarization phenomena, the result causes the interference modulations degree to descend, thereby reduces the signal to noise ratio (S/N ratio) of the spectrum picture that obtains.Its four, the nonlinear problem of optical path difference and fringe position.Transflective and transmission-type interference structure adopt the amplitude-splittine interference principle, therefore must adopt a fourier transform lens just can make optical path difference and fringe position linear in optical interference circuit, and this has also increased the complexity of system.

List of references

[1] Liao Ningfang, based on the wide image processing system of interfere type Fourier transform imaging bands of a spectrum, national 863-13 theme project 2002AA135040 research report, 2004.

[2] Chu Jianjun, transform imaging spectral technology research during Fourier, Beijing Institute of Technology's doctorate paper, 2002..

[3]R.G.Sellar，J.B.Rafert，The?effects?of?aberrations?on?spatially?modulated?Fourier?transformspec-trometers.Opt.Engng.，1994，33(16)：3087～3092.

[4]R.G.ellar，J.B.Rafert，Fourier?transform?imaging?spectrometer?with?a?single?toroidal?optic.Appl.Opt.，1995，34(16)：2931～2933.

[5]J.B.Rafert，R.G.Sellar，J.H.Blatt，Monolithic?Fourier?transform?imaging?spectrometer.Appl.Opt.，1995，34(31)：7228～7230.

[6]P.D.Hammer，F.P.J.Valero，D.L.Peterson，An?imaging?interferometer?for?terrestrial?remote?sensing.Proc.SPIE，1993，1937：244～255.

[7]W.H.Smith，P.D.Hammer，Digital?array?scanned?interferometer：sensors?and?results.Appl.Opt.，1996，35(16)：2902～2909.

[8] refined Zhao in Xiangli preserves Chang Yang to build the former new brilliant high upright honest and tolerant Yuan Yan of people king of peak Wang Wei, high sensitivity inteference imaging spectral apparatus, Chinese patent: 99256131.

[9] refined Zhao in Xiangli preserves Chang Yang to build the former new brilliant high upright honest and tolerant Yuan Yan Wang Wei of people king in peak, a kind of interference imaging spectral technology and device thereof, Chinese patent: 99115952.

[10] open refined Zhao in pure people Xiangli and preserve Chang Yang to build the peak, microminiature stable state polarization interference imaging spectrometer, Chinese patent: 01213109.

[111 pure people, the big visual field of stable state polarization interference imaging spectrometer, Chinese patent: 01213108.

[12] the refined poplar in Xiangli is built peak Ruan Ping and opens pure people Wang Wei, polarization-type interference imaging spectral instrument, Chinese patent: 99256129.

[13]Integrated?optics?in?an?electrically?scanned?imaging?Fourier?transform?spectrometer.Patent?Number：US4523846.

[14]High?Etendue?Imaging?Fourier?Transform?Spectrometer.Patent?Number：US5777736.

[15]Compact?all-reflective?imaging?spectrometer，Patent?Number：US5260767

Summary of the invention

For the problem that the Transflective that overcomes existing Fourier transform imaging spectrometer or transmission-type beam splitting interference structure exist, the present invention proposes the total reflection type Fourier transform imaging spectrometer of a kind of employing Fresnel (Fresnel) double mirror as interference part.The two-sided catoptron of Fresnel is a kind of wavefront-splitting interference device of classics, compares with other amplitude-splittine interference device, and Fresnel double-mirror has the characteristics of total reflection, therefore can be operated in the wavelength band from the visible light to the thermal infrared.Merit attention, the interference modulations degree of the two-sided catoptron of Fresnel and slit width have necessarily gets in touch, therefore in traditional non-imaging-type Fourier transform imaging spectrometer, Michaelson (Michelson) interferometer does not for example generally adopt the two-sided mirror interference structure of Fresnel and has adopted the amplitude-splittine interference structure.Yet, the imaging spectrometer that the present invention relates to has higher requirements to spatial resolution, must adopt a slit to limit instantaneous field of view (IFOV), therefore, adopt the interference structure of the two-sided catoptron of Fresnel can the interference modulations degree of imaging spectrometer not to be impacted.

Fourier transform imaging spectrometer of the present invention has the light channel structure of total reflection.Whole device partly is made up of reflective preposition telescope, entrance slit, reflective collimating mirror, Fresnel double-mirror, reflective cylindrical mirror, focus planardetector and signal acquiring processing system etc.Preposition telescope is imaged on remote linear target on the entrance slit, and this is equivalent in the space remote sensing system, imaging on the entrance slit of interference system perpendicular to pushing away the remote one dimension linear target of sweeping direction.Collimating mirror arrives the two-sided mirror surface of Fresnel to the emergent light parallel projection of entrance slit; The surface gold-plating of Fresnel double-mirror is to realize the wide spectral reflection characteristic of visible light to thermal infrared; After the Fresnel double-mirror reflection, be broken down into the light beam that two bundles have certain angle of cut by the Shu Guang that sends on the slit; The bus of cylindrical mirror is vertical mutually with the intersection of Fresnel double-mirror, therefore can focus on detector surface to the one dimension gray level image of slit, does not hinder the two-beam of Fresnel double-mirror to produce another dimension interference fringe at detector surface simultaneously again.Adopt spectral response range to be respectively visible light and near infrared (VIS﹠amp; NIR, 0.4-1 μ m), the focus planardetector of short-wave infrared (SWIR, 1-5 μ m) and LONG WAVE INFRARED (LWIR, 8-14 μ m), just can realize the interference fringe image signals collecting from the visible light to the infrared band respectively.Signal acquiring processing system is made up of preposing signal process circuit, video capture card, microcomputer system and IO interface etc., signal to focus planardetector output carries out digital collection and processing, and finish by the Fourier transform of spatial domain to frequency domain, obtain the spectral distribution of each dot image on the slit at last.The spectrum picture of a complete width of cloth two dimension target image (being the spectral image data cube) can be by along producing with pushing away of slit vertical direction (push-broom) process of sweeping.

Apparatus of the present invention have that spectral range is wide, luminous flux is big, signal noise ratio (snr) of image is high and the optical texture characteristic of simple, are particularly suitable for the Hyperspectral imager in Aero-Space remote sensing field.

Description of drawings

Fig. 1 is the light channel structure signal of total reflection type Fourier transform imaging spectrometer of the present invention.

Fig. 2 is that Fresnel double-mirror interference figure of the present invention produces the principle signal.

Fig. 3 is the light path principle signal of Fresnel double-mirror of the present invention, cylindrical focusing mirror, focus planardetector.

Fig. 4 is a signal acquiring processing system principle of the present invention.

Fig. 5 is the 1st embodiment schematic diagram.

Fig. 6 is the 2nd embodiment schematic diagram.

Primary structure is among the figure: the preposition telescope of 1-, 2-slit, the reflective collimating mirror of 3-, 4-Fresnel double-mirror, the reflective cylindrical mirror of 5-, 6-focus planardetector, 7-signal acquiring processing system.

Embodiment

The Fourier transform imaging spectrometer of the present invention's narration is characterized in adopting in optical system one group of Fei Nieer double mirror as the beam splitting interference part, thereby realizes the total reflection characteristic of whole optical texture.Now principle of work of the present invention is described as follows in conjunction with Fig. 1, Fig. 2, Fig. 3, Fig. 4 and Fig. 5.

As shown in Figure 1, apparatus of the present invention are made up of reflective preposition telescope 1, slit 2, reflective collimating mirror 3, Fresnel double-mirror 4, reflective cylindrical mirror 5, focus planardetector 6 and signal acquiring processing system 7.

Preposition telescope 1 is imaged on remote thread on the entrance slit 2, and this is equivalent in the space remote sensing system, imaging on the entrance slit of interference system perpendicular to the linear target that pushes away the remote ground of sweeping direction.After slit 2, reflective collimating mirror 3 projects Fresnel double-mirror 4 surfaces to the emergent light of slit; After Fresnel double-mirror 4 reflections, a branch of light of slit 2 outgoing is broken down into the light that two bundles become certain angle of cut mutually; This two-beam projects focus planardetector 6 surfaces after focusing on through reflective cylindrical mirror 5 again, and forms the gray level image distribution of the distribution of one dimension interference fringe and another dimension simultaneously on focus planardetector 6 surfaces.Wherein, the bus of reflective cylindrical mirror 5 is vertical mutually with the intersection of Fresnel double-mirror 4; The direction of interference fringe is parallel with reflective cylindrical mirror 5 buses.Focus planardetector 6 adopts has the senser element of wide spectral response to realize the signals collecting of wide spectral band.Signal acquiring processing system 7 is made up of preposing signal process circuit, video capture card, microcomputer system and IO interface etc., change the video signal of focus planardetector into data image signal, and implement by the Fourier transform of spatial domain to frequency domain, just can obtain along the spectral distribution of the one dimension target image of slit direction distribution.Spectral distribution of another dimension target image can produce by pushing away inswept journey, pushes away that to sweep direction vertical with slit direction.

The principle of interference of Fresnel double-mirror 4 as shown in Figure 2.Two plane mirror S in the Fresnel double-mirror ₁, S ₂Between a very little angle of cut θ is arranged, the intersection of two mirrors is vertical with drawing and by the O point.Consider ideal case, suppose that very thin entrance slit is arranged in figure L place, i.e. L place outgoing be the ideal ball ground roll, suppose that again two catoptrons are respectively the ideal plane, the light that is sent by the L point forms two virtual image L that separate after through two catoptrons so ₁And L ₂, they are a pair of relevant imaginary light source, two image positions in 0 be the center of circle, 0L is on the circumference of radius, arc L ₁L ₂Equal 2 θ.According to the wave optics theory, the light wave that the L point sends is through S ₁And S ₂Obtaining two behind the catoptron is spherical wave, and its center is respectively L ₁And L ₂This two is that spherical wave overlaps, and has produced two bundle sphere wave interferences at this lap.Can collect interferogram on arbitrary plane BB in being positioned over interference region.

Because apparatus of the present invention have adopted collimating mirror and wavefront-splitting interference principle in optical interference circuit, therefore optical path difference Δ L that produces on the focal plane and the relation of position of interference fringe ξ are Δ L=2 ξ sin θ, wherein 0 is the angle of cut of Fresnel double-mirror, so optical path difference and position of interference fringe are linear.

Apparatus of the present invention are calculated by following formula at the fringe-width Δ x that the focus planardetector surface produces:

$\mathrm{\Δx}=\frac{\mathrm{\λ}}{2\sin \mathrm{\θ}}---\left(1\right)$

Wherein λ is an optical wavelength, and θ is the angle of cut of Fresnel double-mirror.By (1) formula as can be known, the angle of cut of Fresnel double-mirror is big more, and interference fringe is close more, and is high more to the resolution requirement of detector.

The theoretic spectrally resolved limit of the present invention (is geometric parameter and the detector characteristic that minimum distinguishable wave-number difference δ v) depends primarily on optical system.Computing formula is:

$\mathrm{\δv}=\frac{1}{2{\mathrm{\ξ}}_M\sin \mathrm{\θ}}---\left(2\right)$

ξ wherein _MBe the breadth extreme of detector, 2 ξ _MSin θ is exactly the maximum optical path difference that interference system obtains.By (2) formula as can be known, the overall width of focus planardetector is big more, and then spectral resolution is high more.

Also can release the cutoff wavelength calculating formula of detectable spectrum of the present invention according to (1) formula:

λ _cut-off＝4dsinθ(3)

Wherein d is the CCD unit size.

By (3) formula as can be known, the detector with high pixel density helps the detectable wavelength coverage of expanding system.

Signal acquiring processing system 7 principles of the present invention as shown in Figure 4.The composite video signal that is collected by focus planardetector 6 comprises the one dimension gray level image information and the one dimension interference fringe information of slit, and under the control of CPU, input signal is kept in the internal memory after preposing signal process circuit and A/D conversion process.System handles the signal of preserving, and finishes by the Fourier transform of spatial domain to frequency domain, obtains the spectral distribution of each dot image on the slit and with result's preservation, prepares against further application.CPU can also push away total system and sweep synchronized sampling, thereby makes system obtain the spectral image data cube of complete object.

In the present invention, (v) the Fourier transform by interference fringe distribution I (ξ) obtains spectral distribution B, that is:

B(v)＝FT{I(ξ)}(4)

In computing machine, can adopt the one-dimensional discrete fourier transform algorithm to calculate (4) formula, method is as follows:

Be located at the perpendicular direction of interference fringe on, can obtain the data sequence that fringe intensity distributes:

I(ξ)，ξ＝0，1，2…N-1

Then the Fourier transform formula of I (ξ) is:

$B\left(0\right)=\frac{1}{\sqrt{N}}\underset{\mathrm{\ξ}=0}{\overset{N-1}{\mathrm{\Σ}}}I\left(\mathrm{\ξ}\right)$

$B\left(v\right)=\sqrt{\frac{2}{N}}\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}I\left(\mathrm{\ξ}\right)\cos \frac{(2\mathrm{\ξ}+1)\mathrm{v\π}}{2N}---\left(5\right)$

v＝1，2，3，...N-1

ξ＝0，1，2，3...N-1

Can utilize commercial fast Fourier transform (FFT) software to finish aforementioned calculation at present.

In result of calculation of the present invention, the wavelength location of spectroscopic data is demarcated the following method that adopts:

According to the optical path difference Δ L of apparatus of the present invention and interference fringe at the linear relationship Δ L=2 of the position coordinates ξ of focal plane ξ sin θ as can be known, for the monochromatic light of any setted wavelength, spacing of the interference fringe that it produces in the focal plane equates; Therefore according to the space invariance of Fourier transform as can be known, through behind the Fourier transform, the scale in frequency domain space also is linear, and promptly in the spectroscopic data sequence of result of calculation, wave number value and data sequence number are proportional.Be located in the result of calculation, the spectroscopic data sequence is:

x ₁，x ₂，x ₃，......x _n

The wave number sequence of their correspondences is:

v ₁，v ₂，v ₃，......v _n

Following proportionate relationship is then arranged:

$\frac{v_1-v_2}{x_1-x_2}=\frac{v_3-v_2}{x_3-x_2}=\mathrm{\Λ}\frac{v_n-v_{n-1}}{x_n-x_{n-1}}---\left(6\right)$

Therefore, in Wavelength calibration,, just can determine the beam location of other light according to (6) formula if the start position of known wave number then only needs definite wherein position of spectral line of a known wavelength light.If determined the beam location of two spectral lines, the beam location that the null position that does not then need wave number also can be demarcated whole wave band according to (6) formula.

Implementation example:

In embodiment shown in Figure 51, apparatus of the present invention are sampled to in-plant linear light source (mercury lamp) target, with the position of spectral line of calibration system.Wherein, two of light source main line wavelengths are respectively 4358nm (22946cm ^-1) and 5461nm (18312cm ^-1); Preposition telescope 1 adopts single off-axis parabolic mirror, focal length is 250mm, effective aperture φ 60mm, from wheelbase from 30mm, have good image quality near axis area: slit 2 is positioned near the focus of preposition telescope 1, so the picture of measured target is become on the plane of incidence of slit.Reflective collimating mirror 3 is same to adopt an off-axis parabolic mirror identical with preposition telescope 1, and its focus in object space overlaps with the image planes position of preposition telescope 1, promptly overlaps with the image planes position of slit 2; Become the central area that parallel beam also projects Fresnel double-mirror 4 at a certain angle after the emergent light of slit 2 is collimated.Reflective cylindrical mirror 5 adopts a parabola shaped cylindrical mirror from the axle design, and focal length is 40mm, effective aperture 50 * 50mm, from wheelbase from 30mm; Focus planardetector adopts silicon ccd array (1/2 inch, 768 * 586 pixels), at visible light and near-infrared band (0.4-1.0 μ m) response is arranged.Image pick-up card is the ordinary video image pick-up card of quantified precision 8bit, is connected with computing machine by pci bus.Signal processing system adopts the visual programming technology, finishes functions such as image acquisition, noise processed, FFT conversion, data storage.

If slit width is 0.1mm, then Dui Ying instantaneous field of view angle is:

IFOV＝0.1/250＝0.004rad＝4mrad.

If the angle of cut of Fresnel double-mirror is 0.5 °, the CCD light-sensitive surface receives effective width 2 ξ of interfering beam _MBe 7.68mm, then spectrum (wave number) resolving limit by the optical system restriction is:

Establishing CCD unit size d simultaneously is 9 μ m, and then the detectable cutoff wavelength by the optical system restriction is:

λ _cut-off＝4dsinθ＝4×9000×sin0.5°≈314nm

In embodiment shown in Figure 62, apparatus of the present invention are sampled to one dimension black and white band target, to obtain the spectral image data cube.Wherein, measured target is thrown light on by common halogen tungsten lamp, therefore has the continuous spectrum signal at visible light and near-infrared band; Strip direction is vertical with slit direction; Parameter and Fig. 5 of optical system, focus planardetector are identical.In addition, device shown in Figure 6 also can be to the sampling of infrared band target, and change the band target into isothermal band infrared radiation target source this moment, and focus planardetector is changed into infrared focal plane detector.For example, adopt refrigeration mode PtSi-CCD array (512 * 512 pixel) at the 1-5 mu m waveband; At thermal infrared 8～12 mu m wavebands, adopt refrigeration mode HgCdTe focal plane device or non-refrigeration type infrared focal plane device (512 * 512 pixel).The angle of cut of establishing Fresnel double-mirror equally is 0.5 °, and the single pixel dimension of PtSi-CCD array is 30 μ m, and then the cutoff wavelength of the detectable spectrum of system is:

λ _cut-off＝4×30×sin0.5°≈1.04μm.

Claims (1)

1. a total reflection type Fourier transform imaging spectrometer that adopts Fresnel double-mirror comprises optical texture, focus planardetector (6) and signal acquiring processing system (7); It is characterized in that: in its optical texture, adopt one group of two-sided catoptron of Fresnel (4) to constitute optical interference structure; Fresnel double mirror (4) is by two plane mirror s with certain angle theta ₁, s ₂Form; Optical texture also comprises a cover reflective preposition telescope (1), slit (2), reflective collimating mirror (3), reflective cylindrical mirror (5); Slit (2) is positioned on the focal plane of preposition telescope (1), and the focus in object space of reflective collimating mirror (3) overlaps with the image planes position of slit (2); Fresnel double mirror (4) is positioned at reflective collimating mirror (3) afterwards; Be provided with reflective cylindrical mirror (5), focus planardetector (6) and signal acquiring processing system (7) behind the Fresnel double mirror (4); Reflective cylindrical mirror (5) bus intersection and the slit (2) with Fresnel double mirror (4) two mirrors respectively is vertical.

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