CN102778300A - Method for stably measuring atmospheric coherence length - Google Patents

Abstract

The invention belongs to the field of optical technology, and particularly relates to a method for stably measuring atmospheric coherence length r0. According to space-time probability consistency characteristic of atmospheric turbulence, multiple pairs of sub-apertures which are in regular polygon symmetrical distribution on a Hartman wave front detector are selected, so as to increase the space statistic probability and simultaneously reduce the sampling time; In order to continue to calculate r0 by use of a formula (1), the method of selecting the multiple pairs of sub-apertures is as follows: the distance between centers of each pair of sub-apertures is strictly the same and simultaneously meets the principle of selecting one pair of sub-apertures, the regular polygon symmetrical distribution of the multiple pairs of sub-apertures is simplified as square distribution under the structure restriction of the Hartman wave front detector; and by utilizing hundreds of light spot array signals of a turbulence wave surface collected within one second, the difference movement of multiple pairs of image points which are in square symmetrical distribution on each light spot array signal is substituted into the formula (1) so as to be subjected to identical statistic average, so that an accurate atmospheric coherence length r0 value with an instant characteristic can be obtained.

Description

The stably measured method of atmospheric coherence length

Technical field

The invention belongs to the adaptive optics field, relate to the adaptively correcting Evaluation on effect parameter of plasmas channel, specifically the atmospheric turbulence intensity parameter is the stably measured method of atmospheric coherence length.

Background technology

10 ~ 20 kilometers atmospheric envelope is arranged on the earth surface vertical direction,, promptly cause air index inhomogeneous, thereby form the dynamic disturbances phenomenon of atmospheric turbulence before light wave because the inhomogeneous variation of temperature makes air local density different.When utilizing astronomical telescope to extraterrestrial target observation imaging, the target light wave that astronomical telescope receives receives the dynamic disturbances of atmospheric turbulence and produces wavefront distortion in various degree, has a strong impact on image quality.Therefore, self-adaptation wavefront correction system becomes the telescopical necessary facility of the above bore of meter level.In order to estimate the ability of self-adaptation wavefront correction system reply atmospheric turbulence, the measurement of atmospheric turbulence intensity is the important topic in adaptive optics field always.

Atmospheric coherence length r ₀Numerical value can express the spatial coherence that light beam transmits and the comprehensive strength of atmospheric turbulence dynamic disturbances in atmospheric turbulence.Atmospheric coherence length r ₀Value more little, the dynamic disturbances Shaoxing opera that characterizes atmospheric turbulence is strong, span is generally between several centimetres to tens centimetres.r ₀Being defined as of value: optical wave-front is at atmospheric coherence length r ₀The root-mean-square value RMS that the position rises and falls mutually in the sub-aperture in other words conj.or perhaps is 1rad for the round territory of diameter.When the diameter of optical wave-front surpasses atmospheric coherence length r ₀The value yardstick the time, the amount of distortion of optical wave-front will reduce the imaging resolution of optical receiving system.

Usually adopt two kinds of methods to measure atmospheric coherence length r ₀: a kind ofly be through the measurement of Atmospheric Turbulence refractive index structure parameter and then obtain atmospheric coherence length r ₀, need the process long-term observation just can obtain the statistical law of refractive index structure parameter; Another kind is to obtain atmospheric coherence length r through the phase information of measuring distorted wavefront ₀, be exactly that so-called difference asterism is as the method for movement (DIMM), by extensive employing.

The difference asterism is proposed in nineteen sixty by Stock and Keller as the method for movement (DIMM); Its measuring principle is: a baffle plate is placed in the entrance pupil position at astronomical telescope; Opened two sub-aperture on the baffle plate, the distance between two sub-aperture is greater than sub-aperture diameter.Can be divided into two independently asterism pictures after arriving the CCD Observation camera from the point target light beam of the plasmas channel other end through two sub-aperture.Within a certain period of time, the variation of relative distance just can calculate atmospheric coherence length r between two picture points that caused by the atmospheric turbulence dynamic disturbances through real time record ₀Value.F.Roddie derives and calculates atmospheric coherence length r on the basis of Fried theoretical research ₀Fundamental formular [F.Roddier, The effects of atmospheric turbulence in optical astronomy, [J] .Prog.Optics, 1981,19:281 ~ 376]:

${\mathrm{<figure-callout\; id="0"\; label="r"\; filenames="HDA00001941412100021.png"\; state="\{\{state\}\}">r</figure-callout>}}_0={\left[\frac{{2f}^2{l}^2({0.358D}^{-1/3}-{0.242Z}^{-1/3})}{<{\mathrm{<figure-callout\; id="2"\; label="Z\; q"\; filenames="HDA00001941412100011.png"\; state="\{\{state\}\}">Z</figure-callout>}}_q^{}>-{<Z_q>}^2}\right]}^{3/5}$

Wherein, f is the focal length of measuring system, and λ is for receiving the centre wavelength of light, and D is the diameter in sub-aperture, and Z is the center distance in two sub-apertures, and Z>>r ₀, Z _qBe the imaging point barycenter spacing in two sub-apertures,<>is time statistical average symbol.Picture point center-of-mass coordinate (c _x, c _y) ask the calculation method according to [Francois Roddier, Adaptive optics in astronomy, Cambridge University Press, 1999, Part two, pp99]:

$c_x=\frac{{\mathrm{\&Sigma;}}_{i,j}{x}_{i,j}{I}_{i,j}}{{\mathrm{\&Sigma;}}_{i,j}{I}_{i,j}},$ $c_y=\frac{{\mathrm{\&Sigma;}}_{i,j}{y}_{i,j}{I}_{i,j}}{{\mathrm{\&Sigma;}}_{i,j}{I}_{i,j}}---\left(2\right)$

I wherein, j are the sequence numbers that rectangular coordinate that the CCD camera pixel constitutes is fastened pixel, x _{I, j}With y _{I, j}Be respectively (i, j) two of pixel coordinate components, I _{I, j}Be (i, j) light intensity of pixel.

In fact adopt the Shack-Hartmann wave front detector can be to r ₀Carry out the DIMM method measurement [Rao Changhui, Jiang Wenhan, Ling Ning. use Shack-Hartmann wavefront sensor measurement of Atmospheric Turbulence parameter [J]. optics journal, 2000,20 (9): 1201 ~ 1207].Use the Shack-Hartmann wave front detector of about KHz SF, select distance two sub-aperture enough far away on it, the position of this two sub-aperture is centrosymmetric on the reception corrugated usually.Array of light spots during measurement on continuous acquisition 10s, about 10000 width of cloth plasmas channels is only considered the asterism picture element heart distance of selecteed two sub-aperture on each width of cloth, then according to (1) formula statistical computation r ₀

But, the atmospheric coherence length r that adopts the DIMM method to record in some moment ₀Value rises to two, three centimetres suddenly through regular meeting after the several seconds, also can rise to nearly ten centimetres accidentally.This phenomenon does not conform to the velocity characteristic of atmospheric turbulence.Analyzing its reason, should be that statistic causes inadequately, because the fluctuating severe of wavefront all is different on each orientation, the DIMM method is only added up the motion of two sub-aperture picture point barycenter on the fixed position, wants statistics and must take a long time comprehensively.Also be difficult to guarantee r if add up 10,000 times ₀The accuracy of measuring, explain 10,000 times statistic not enough; But increase statistic again, also might cause the sampling time long, surpass r ₀Keep the constant time, on average fall r ₀Variation.So traditional measurement atmospheric coherence length r ₀The DIMM method be defective.

2000 rich adopts three holes, i.e. three couples of asterism aberration componental movement methods [Rao Ruizhong, Wang Shipeng, Liu Xiaochun in auspicious; Deng. the experimental study of the laser beam drift in the turbulent atmosphere [J]. Chinese laser; 2000,27 (11): 1011-1015.], attempt remedying a pair of asterism not comprehensive as movement statistics.2007 rich auspicious in seminar reported again and adopted vertical each other two pairs of holes (four holes), i.e. six couples of asterism aberration componental movement methods [Huang Honghua, Yao Yongbang, Rao Ruizhong; The method research [J] of four hole Differential Image motion measurement atmospheric coherence lengths. the light laser and the particle beams, 2007,19 (3): 357-360.]; And in article in 2010 [Ni Zhibo, Huang Honghua, Huang Yinbo; Red legend is got over, and Rao Ruizhong [J] .Dome effect is to four hole atmospheric coherence length instrument system Effect on Performance, the light laser and the particle beams; 2010,22 (11): 2551-2555.] added up the difference result of 6 couples of asterism picture * 200 width of cloth figure, the r that records ₀Pace of change is significantly slowed down, about 0.6～0.7cm/min, but still can't confirm whether this statistic is abundant, i.e. r ₀Variation whether true, whether the time dependent influence of the local opposite sex of wavefront is arranged; The center distance in per two sub-apertures is not quite similar in these 6 pairs of apertures in addition, must transform the statistical calculation method of formula (1), and calculated amount is increased.

Summary of the invention

The present invention is in order to overcome atmospheric coherence length r ₀The defective of statistical method; Many antithetical phrases aperture that regular polygon is symmetrically distributed is chosen in proposition on the spot array that the Shack-Hartmann wave front detector records; Significantly increase assemblage statistics probability; Keep the statistical calculation method of formula (1) effective simultaneously, purpose provides a kind of accurate and easy atmospheric coherence length r ₀The stably measured method.

The present invention is detailed below.The Shack-Hartmann wave front detector is docked with an astronomical telescope optics, make the light beam of any fixed star that telescope receives can be unified and receive identical, the parallel incident Shack-Hartmann of bore wave front detector with the Shack-Hartmann wave front detector.According to the consistent characteristic of the space-time probability of atmospheric turbulence, on the spot array that the Shack-Hartmann wave front detector records, choose many antithetical phrases aperture that regular polygon is symmetrically distributed, to increase the spatial statistics probability, reduce the sampling time simultaneously.In order to continue to utilize formula (1) to calculate r ₀, the method for choosing many antithetical phrases aperture is to keep the Z value in the formula (1) constant, promptly the center distance in each antithetical phrase aperture is all strict identical, and much larger than r ₀Many antithetical phrases aperture that regular polygon is symmetrically distributed under the restriction of Shack-Hartmann wave front detector structure so just can only be reduced to square as shown in Figure 1 or sexangle and distribute, and selects 10～14 antithetical phrase apertures on it.The SF of Shack-Hartmann wave front detector must not be less than 500Hz, the spot array signal on continuous acquisition 300～500 width of cloth turbulent flow corrugateds.With on 300～500 width of cloth arrays each to the barycenter spacing Z of picture point _qAll carry out statistical average in the substitution formula (1), draw error less than atmospheric coherence length r 3%, that have instant characteristic ₀Value.

Description of drawings

Fig. 1 measures atmospheric coherence length r for utilizing Atmosphere Turbulence Simulator among the present invention ₀Laboratory experiment light path synoptic diagram.Wherein 1 is the Halogen lamp LED pointolite, and 2 is the monochromatic filter of 785nm wavelength, and 3 is first lens; With the optical alignment of pointolite is the suitable directional light of diameter, and 4 is the rotation phase board of Atmosphere Turbulence Simulator, and 5 is aperture diaphragm; 6 is second lens, and 7 is the 3rd lens, the two composition bundle lens combination that contracts; 8 is the Shack-Hartmann wave front detector; Light beam after the disturbance of unified reception Atmosphere Turbulence Simulator, 9 is the computing machine that links to each other with Shack-Hartmann wave front detector 8, reads and store the spot array diagram data of being gathered.

Fig. 2 among the present invention on the spot array figure of Shack-Hartmann the choosing method of 12 pairs of points.On two vertical direction of spot array, select the i.e. 12 times of r of 12 hot spots of spacing respectively ₀6 pairs of hot spots, form the asterism picture in 12 antithetical phrase apertures of square profile, with the paired luminous point of alphabetic flag: a and a ', b and b ', c and c ', d and d ', e and e ', f and f ', g and g ', h and h ', i and i ', j and j ', k and k ', l and l '.

Fig. 3 chooses 1 pair of point and 12 pairs of points respectively, the r of acquisition in the specific embodiment of the invention on the spot array figure of Shack-Hartmann ₀With the change curve of statistic, wherein curve A is the result of 1 pair of point, and curve B is the result of 12 pairs of points.

Embodiment

1) builds measurement r according to Fig. 1 ₀The experiment light path.Wherein the spherical light wave that sends of Halogen lamp LED pointolite 1 is 785nm through wavelength; Become monochromatic light through monochromatic filter 2;, first lens, 3 collimations incide on the rotation phase board 4 of Atmosphere Turbulence Simulator after being directional light again; Aperture diaphragm 5 is arranged on the clear aperture place near the rotation phase board of Atmosphere Turbulence Simulator; The aperture of aperture diaphragm 5 is set to 10mm; Light beam through aperture diaphragm 5 enters on the logical light window incircle of Shack-Hartmann wave front detector 8 through second lens 6, the 3rd lens 7 2.5 times of parallel beams that become the 4mm bore of bundle that contract again, and computing machine 9 links to each other with Shack-Hartmann wave front detector 8, reads and store the spot array diagram data of being gathered.

2) Atmosphere Turbulence Simulator described in the step 1) is the Lexltek Company products, confirms atmospheric coherence length r through the clear aperture on the control bit phase-plate ₀, confirm that through control bit phase-plate velocity of rotation the turbulent flow frequency is Green's martial virtue frequency: at first setting turbulent flow centre wavelength is 785nm, setting the telescope bore then is 500mm, atmospheric coherence length r ₀Being set at 5.0cm, is 10mm at the spot diameter that finds on the service manual of this product on the corresponding positions phase-plate, and selecting the turbulent flow frequency is Green's martial virtue frequency 31Hz, and on the service manual of this product, finding corresponding positions phase-plate rotating speed is 20 commentaries on classics/min.

3) aperture of aperture diaphragm 5 can be regulated in the 15mm scope at 1mm continuously, can control the beam diameter that incides on the Atmosphere Turbulence Simulator rotation phase board 4.

4) Shack-Hartmann wave front detector 8 frame frequencies are 1020Hz; Photo-sensitive cell is the MicroVista-NIR back illumination formula CMOS camera of Intevac Photonics company; The pixel count of this camera reaches 1280 * 1024, and the lenticule number is 20 * 20, with the grid arrayed, corresponding 7 * 7 pixels of each lenticule; The pixel count that uses the CMOS camera altogether is 140 * 140, and corresponding window is 4mm * 4mm.

5) first lens 3, second lens 6, the 3rd lens 7 are achromatic doublet, and wherein the bore of first lens 3 is 40mm, and focal length is 270mm, and making the spot diameter that arrives on the phase board is 10mm, is the wavefront of 500mm with the simulation bore; The bore of second lens 6, the 3rd lens 7 is respectively 40mm and 25mm, and focal length is respectively 250mm, 100mm, and bundle is 2.5 times so that light beam contracts, and its bore is identical with the clear aperture of Shack-Hartmann wave front detector 8.

6) behind the opening point light source 1, spot array such as Fig. 2 that Shack-Hartmann wave front detector 8 obtains select the i.e. 12 times of r of 12 hot spots of spacing respectively on two vertical direction of spot array ₀6 pairs of hot spots, form the asterism picture in 12 antithetical phrase apertures of square profile, like the paired luminous point of Reference character among Fig. 2.

7) behind the unlatching Atmosphere Turbulence Simulator rotation phase board 4, continuous acquisition 10000 width of cloth spot array data are chosen 1 pair of point and 12 pairs of points respectively on spot array figure, calculate the barycenter spacing Z of every pair of asterism picture _qIn the substitution formula (1), the r of acquisition ₀Change curve with statistic is as shown in Figure 3, and wherein curve A is the result of 12 pairs of points, and curve B is the result of 1 pair of point, finds out the r that 300 width of cloth spot array data statisticss are come out in the curve A ₀Restrain, be 5.1cm, with r ₀Setting value 5.0cm compares has only 2% error, and curve B then just restrains later on to 6500 width of cloth, and converges to 5.5cm, with r ₀Setting value 5.0cm compares has 10% error.

Claims (3)

1. atmospheric coherence length r ₀The stably measured method; It is characterized in that the Shack-Hartmann wave front detector is docked with an astronomical telescope optics, make the light beam of any fixed star that telescope receives can be unified and receive identical, the parallel incident Shack-Hartmann of bore wave front detector with the Shack-Hartmann wave front detector; On the spot array that the Shack-Hartmann wave front detector records, choose 10～14 antithetical phrase apertures that square symmetry distributes,, reduce the sampling time simultaneously to increase the spatial statistics probability; Utilize formula $r_0={\left[\frac{{2f}^2{l}^2({0.358D}^{-1/3}-{0.242Z}^{-1/3})}{<Z_q^2>-{<Z_q>}^2}\right]}^{3/5}$ Calculate r ₀, the f in the formula is the focal length of measuring system, and λ is for receiving the centre wavelength of light, and D is the diameter in sub-aperture, and Z is the center distance in every antithetical phrase aperture, and Z>3r ₀, Z _qBe the imaging point barycenter spacing in two sub-apertures,<>is time statistical average symbol; The method of choosing many antithetical phrases aperture is to keep the Z value in the formula constant, and promptly the center distance in each antithetical phrase aperture is all strict identical; The SF of Shack-Hartmann wave front detector must not be less than 500Hz, the spot array signal on continuous acquisition 300～500 width of cloth turbulent flow corrugateds; With on 300～500 width of cloth arrays each to the barycenter spacing Z of picture point _qAll carry out statistical average in the substitution formula, draw atmospheric coherence length r stable, that have instant characteristic ₀Value.

2. atmospheric coherence length r according to claim 1 ₀The stably measured method, it is characterized in that: Shack-Hartmann wave front detector frame frequency is 1020Hz.

3. atmospheric coherence length r according to claim 2 ₀The stably measured method, it is characterized in that: the spot array figure that on the Shack-Hartmann wave front detector, obtains, select the i.e. 12 times of r of 12 hot spots of spacing on its two vertical direction respectively ₀6 pairs of hot spots, form the asterism picture in 12 antithetical phrase apertures of square profile, continuous acquisition 300 width of cloth spot array data are with the barycenter spacing Z of every pair of asterism picture _qIn the described formula of substitution claim 1, statistical computation goes out r ₀

Images