Research On Improving Dynamic Range Of Pyramid Wavefront Sensor

As known,adaptive optics system is used to measure and compensate the atmospheric turbulence received by telescope in real time.Thus the diffraction limited imaging quality of the telescope is recovered.Due the improvements in astronomy and military,observe targets for telescope become dimmer.The classic Shack-Hartmann wavefront sensor(SHWFS)divides the incoming ray into dozens or even hundreds sub-apertures,thus limits the sensing magnitude of SHWFS.However large the telescope aperture is,the limiting sensing magnitude can be no more than 5.5mag.The recently proposed Pyramid wavefront sensor(PWFS)is able to sense dimmer targets than SHWFS.PWFS converges the incident beam onto the apex of a pyramid prism,thus divides the beam into only four sub-beams and forming four images of system pupil.Wavefront turbulence offsets the apex spot from the pyramid tip,it results in change of both total and interior intensity of the pupil images.The turbulence can be calculated according to the difference of pupil images intensity.However,the dynamic range of PWFS is relatively small.When the incident turbulence becomes larger,especially when the tip-tilt increases,the apex spot entirely leaves the pyramid tip and only one pupil image is illuminated.Thus PWFS is applied only in closed-loop adaptive systems,while no report of PWFS applied in open-loop system is found.Open-loop control is the development trend of adaptive system,it requires larger dynamic range and higher sensing accuracy of the wavefront sensor.Concerning the small dynamic range of PWFS,we considered replacing the apex spot offset into exposure time difference to quantitatively reduce the intensity difference of pupil images,thus enlarging the dynamic range of PWFS.A fast-steering mirror was added to produce circular modulation of the focus spot on pyramid.Radius of the circular modulation should be large enough to ensure that the entire spot goes across four pyramid facets in turn.The tilt of incoming wavefront offsets the center of circular path,it makes the spot moving time on each facet different,so is the exposure time of each pupil image.Thus modulates the intensity of pupil images.The reconstruction method is still available for the pupil images intensity differences are still proportional to the wavefront local tilt.The tilt is calculated using the total intensity difference of four pupil images while high-order abberation is reconstructed using intensity difference on corresponding subapertures of each image.Principle of the modulation requires strict conjugation relation between the pupil image and system pupil,for it ensures the position of pupil images independent with the dynamic modulation.The dynamic range of PWFS was extended to 12? from 1?,? = 780 nm,meeting the requirement of compensating real turbulence.However,the modulation of PWFS induces following problems.When the apex spot crosses the pyramid edges,bright bands are produced by the cylindrical platform on the edges.Adding with the pupil images,these bright bands induces high-order error into reconstructed wavefront.The offset of pupil images should be no bigger than 3 pixels on camera,or the pupil images cannot be located precisely.Due to the axial chromatic aberration induced by the pyramid and diffraction of image edges,a blur of a dozen pixels around the pupil image is produced.And the incident turbulence is so severe that the approximation in small turbulence conditions is no longer available.Moreover,the modulation changes the source of intensity difference to exposure time of images,the response matrix of modulated PWFS should be calculated.As for improving the sensing accuracy,the precise calibration process of PWFS was studied.Impact on sensing signal from freedom of focus lens,pyramid prism,relay lens and sensor camera was analyzed.Precise calibration was conducted based on the analytical result,the non-common path abberation was reduced and noise induced by interference among pupil images was weakened.A precise method of extraction and alignment of pupil image in PWFS was proposed,improving the signal extraction precision to sub-pixel level.Moreover,the calculation of frame extraction was cut by10%.A testbench for sensing and correcting static abberation in laboratory was established.Experimental results showed that the measurement error in open-loop was reduced by 60%,convergence speed of closed-loop correction was increased and the residual correction error was reduced by 15%.A response matrix for large incident turbulence was derived based on physical optics and by applying this matrix the reconstruction accuracy of modulated PWFS was improved by about 50%,residual error of corrected wavefront was under ?/20.A testbench for correction on static abberation using PWFS and LCSLM was established and verified that the modified PWFS met the requirement of open-loop correction of static abberation.