Dayu Li

Changchun Institute of Optics, Fine Mechanics and Physics, Hsin-ching, Jilin Sheng, China

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Publications (20)49.96 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a novel method to measure the interaction matrix of liquid-crystal adaptive optics systems, by applying least squares method to mitigate the impact of measurement noise. Experimental results showed a dramatic gain in the accuracy of interaction matrix, and a considerable improvement in image resolution with open loop adaptive optics correction.
    Optics Express 06/2014; 22(12):14221-14228. · 3.55 Impact Factor
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    ABSTRACT: To process real-time wavefront signal in our liquid crystal adaptive optics system (LCAOS), a real-time wavefront processor (RTWP) with graphics processing unit (GPU) accelerator is developed. A series of parallel acceleration algorithm is implemented to reconstruct wavefront, compute gray map and process lookup table (LUT). Especially, an algorithm by using symmetry of Zernike polynomial is designed to compute gray map and it provides a speedup of 3.16. Finally, the total computing time in our RTWP is 109 μs and temporal bandwidth in our LCAOS is 45 Hz; it is shown that our RTWP has the ability to real time process wavefront signal in our LCAOS.
    Optics Communications 01/2014; 316:211–216. · 1.44 Impact Factor
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    ABSTRACT: The residual aberrations of the adaptive optics retinal imaging system will decrease the quality of the retinal images. To overcome this obstacle, we found that the optical transfer function (OTF) of the adaptive optics retinal imaging system can be described as the Levy stable distribution. Then a new method is introduced to estimate the OTF of the open-loop adaptive optics system, based on analyzing the residual aberrations of the open-loop adaptive optics system in the residual aberrations measuring mode. At last, the estimated OTF is applied to restore the retinal images of the open-loop adaptive optics retinal imaging system. The contrast and resolution of the restored image is significantly improved with the Laplacian sum (LS) from 0.0785 to 0.1480 and gray mean grads (GMG) from 0.0165 to 0.0306.
    Optics Communications 07/2013; 300:178–182. · 1.44 Impact Factor
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    ABSTRACT: Digital overdriving technique is used to enhance response performance of liquid crystal device for wave front correction, combined with electro-optic effect. Generally, using phase wrapping technique, liquid crystal wave front correction only needs 1λ phase retardation. In this new driving scheme, it was increased to 1.2λ and only the phase retardation from 0 to 1λ was used for real correction. Different from conventional method, transition grey level is needed for each grey level as to accelerate its response. This method could enhance the response time up to 50% and unify all the grey-to-grey responses to one equal value. It can be easily implemented into every conventional Liquid Crystal devices without any additional hardware modification. A liquid crystal on silicon device with synthesised liquid crystal material by our lab was used to demonstrate its feasibility. After optimisation, the response time was reduced from nearly 3.3 ms down to 1.55 ms.
    Liquid Crystals 01/2013; 40(6). · 1.96 Impact Factor
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    ABSTRACT: As a wavefront corrector, the phase-only liquid crystal spatial light modulator has been widely using in adaptive optics systems. However, the unmodulated diffracted beam of the modulator will affect the light spot centroid detection accuracy of a Shack–Hartmann wavefront sensor and decrease the image quality after correction. In this paper, we have diminished the effect by introducing a modified weight algorithm in our closed-loop adaptive optics system. The Strehl ratio of the image is higher than 0.8 after correction, even when the wavefront aberration is larger than 3 μm. The correction precision and image quality are both improved significantly.
    Journal of optics 10/2012; 14(10):105705. · 2.01 Impact Factor
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    ABSTRACT: An open-loop adaptive optics (AO) system for retinal imaging was constructed using a liquid crystal spatial light modulator (LC-SLM) as the wavefront compensator. Due to the dispersion of the LC-SLM, there was only one illumination source for both aberration detection and retinal imaging in this system. To increase the field of view (FOV) for retinal imaging, a modified mechanical shutter was integrated into the illumination channel to control the size of the illumination spot on the fundus. The AO loop was operated in a pulsing mode, and the fundus was illuminated twice by two laser impulses in a single AO correction loop. As a result, the FOV for retinal imaging was increased to 1.7-deg without compromising the aberration detection accuracy. The correction precision of the open-loop AO system was evaluated in a closed-loop configuration; the residual error is approximately 0.0909λ (root-mean-square, RMS), and the Strehl ratio ranges to 0.7217. Two subjects with differing rates of myopia (-3D and -5D) were tested. High-resolution images of capillaries and photoreceptors were obtained.
    Journal of Biomedical Optics 02/2012; 17(2):026001. · 2.75 Impact Factor
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    ABSTRACT: In this paper, we summarize the researches of liquid crystal (LC) adaptive optics(AO) in astronomical applications. Dual frequency and nematic LC materials were reported to be used in astronomical observations. However, nematic LC with its simple producing and driving method and multi-gray-level is promising in the application. We have investigated the nematic LC AO system and promoted its application for nearly ten years. Here, we reported our newest nematic LC AO system with bandwidth result of -3dB is 54.4 Hz, 0 dB bandwidth is 79.1Hz.
    Optoelectronics and Microelectronics (ICOM), 2012 International Conference on; 01/2012
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    ABSTRACT: Using the liquid crystal spatial light modulator (LC-SLM) as the wavefront corrector, an open-loop adaptive optics (AO) system for fundus imaging in vivo is constructed. Compared with the LC-SLM closed-loop AO system, the light energy efficiency is increased by a factor of 2, which is helpful for the safety of fundus illumination in vivo. In our experiment, the subjective accommodation method is used to precorrect the defocus aberration, and three subjects with different myopia 0, -3, and -5 D are tested. Although the residual wavefront error after correction cannot to detected, the fundus images adequately demonstrate that the imaging system reaches the resolution of a single photoreceptor cell through the open-loop correction. Without dilating and cyclopleging the eye, the continuous imaging for 8 s is recorded for one of the subjects.
    Journal of Biomedical Optics 01/2010; 15(4):046009. · 2.75 Impact Factor
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    ABSTRACT: To generate time-evolving atmosphere turbulence in real time, a phase-generating method for our liquid-crystal (LC) atmosphere turbulence simulator (ATS) is derived based on the Fourier series (FS) method. A real matrix expression for generating turbulence phases is given and calculated with a graphic processing unit (GPU), the GeForce 8800 Ultra. A liquid crystal on silicon (LCOS) with 256x256 pixels is used as the turbulence simulator. The total time to generate a turbulence phase is about 7.8 ms for calculation and readout with the GPU. A parallel processing method of calculating and sending a picture to the LCOS is used to improve the simulating speed of our LC ATS. Therefore, the real-time turbulence phase-generation frequency of our LC ATS is up to 128 Hz. To our knowledge, it is the highest speed used to generate a turbulence phase in real time.
    Optics Express 05/2009; 17(9):7259-68. · 3.55 Impact Factor
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    ABSTRACT: A nematic liquid crystal adaptive optics system (NLC AOS) was assembled for a 2.16-m telescope to correct for atmospheric turbulence. LC AOS was designed and optimized with Zemax optical software. Second, an adaptive correction experiment was performed in the laboratory to test the performance of the NLC AOS. After the correction, the peak to valley (PV) and root mean square (RMS) of the wavefront were down to 0.2 lambda (lambda=633 nm) and 0.05 lambda, respectively. Finally, the star of Pollux (beta Gem) was tracked using the 2.16-m Reflecting Telescope, and real time correction of the atmospheric turbulence was performed with the NLC AOS. After the adaptive correction, the average PV and RMS of the wavefront were reduced from 11 lambda and 2.5 lambda to 2.3 lambda and 0.6 lambda, respectively. Although the intensity distribution of the beta Gem was converged and its peak was sharp, a halo still existed around the peak. These results indicated that the NLC AOS only partially corrected the vertical atmospheric turbulence. The limitations of our NLC AOS are discussed and some proposals are made.
    Optics Express 03/2009; 17(4):2530-7. · 3.55 Impact Factor
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    ABSTRACT: A comprehensive noise model about digital camera which is a main component of SHWFS is constructed, including the readout noise, the photon shot noise, the quantization noise and the response un-uniformity. Based on the noise model, the spot centroid errors caused by each kind of noise are analyzed, respectively. And then the synthetic error from all the noise is calculated. The result demonstrates that the limit of the spot centroid accuracy is 1% pixels. At last, the crossing error caused by the high order diffraction spots is analyzed. It is approximately proportional to the secondary spots number. So the structure of the microlens array must be optimized together with the digital camera when designing SHWFS.
    Optics Communications 01/2009; 282(22):4333-4338. · 1.44 Impact Factor
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    ABSTRACT: Adaptive optics systems often work in a closed-loop configuration due to the hysteretic and nonlinearity properties of conventional deformable mirrors. Because of the high-precision wavefront generation and nonhysteretic properties of liquid-crystal devices, the open-loop control becomes possible. Open-loop control is a requirement for advanced adaptive optics concepts. We designed an open-loop adaptive optics system with a liquid-crystal-on-silicon wavefront corrector. This system is simple, fast, and can save much more light compared to conventional liquid-crystal-based closed-loop systems. The detailed principle, construction, and operation are discussed. The 500 m horizontal turbulence correction experiment was done using a 250 mm telescope in the laboratory. The whole system can reach a 60 Hz correction frequency. Evaluation of the correction precision was done at closed-loop configuration, which is 0.2 lambda (lambda=0.633 microm) in peak to valley. The dynamic image under open-loop correction got the same resolution compared to closed-loop correction. The whole system reached 0.68 arc sec resolution capability at open-loop correction, which is slightly larger than the system's diffraction-limited resolution of 0.65 arc sec.
    Applied Optics 09/2008; 47(23):4297-301. · 1.69 Impact Factor
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    ABSTRACT: To correct horizontal turbulences, a nematic liquid crystal wavefront corrector (NLC WFC) with a fast response is used. It can linearly modulate 2pi radian at a wavelength of 633 nm. The closed-loop frequency of the adaptive optics system was originally only 12 Hz. Hence, a control system using the NLC WFC was developed, graphic processing units (GPUs) were used to compute the compensated wavefront, and the driving software for the NLC WFC was optimized. With these improvements, the closed loop frequency increased up to 60 Hz. Finally, the correction of a 500-m horizontal turbulence was performed with this fast adaptive system. After the correction, the averaged peak-to-valley (PV) and root-mean-square (RMS) values of the wavefront were reduced to 0.2 lambda and 0.06 lambda, respectively. The core of a fiber bundle is also resolved with a field angle of 0.68". As the limit of the angular resolution of the telescope is 0.65", the quasi-diffraction limited image is acquired with the closed-loop correction. It is shown that the NLC WFC has the ability to correct weak turbulences.
    Optics Express 06/2008; 16(10):7006-13. · 3.55 Impact Factor
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    ABSTRACT: A collimator with a long focal length and large aperture is a very important apparatus for testing large-aperture optical systems. But it suffers from internal air turbulence, which may limit its performance and reduce the testing accuracy. To overcome this problem, an adaptive optics system is introduced to compensate for the turbulence. This system includes a liquid crystal on silicon device as a wavefront corrector and a Shack-Hartmann wavefront sensor. After correction, we can get a plane wavefront with rms of about 0.017 lambda (lambda=0.6328 microm) emitted out of a larger than 500 mm diameter aperture. The whole system reaches diffraction-limited resolution.
    Applied Optics 04/2008; 47(9):1298-301. · 1.69 Impact Factor
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    ABSTRACT: Based on a simple eye model system, a high resolution adaptive optics retina imaging system was built to demonstrate the availability of using liquid crystal devices as a wave-front corrector for both low and high order aberrations. Myopia glass was used to introduce large low order aberrations. A fiber bundle was used to simulate the retina. After correction, its image at different diopters became very clear. We can get a root mean square (RMS) correction precision of lower than 0.049lambda (lambda=0.63mum) for over to 10 diopters and the modulation transfer function (MTF) retains 511p/mm, which is nearly the diffraction limited resolution for a 2.7mm pupil diameter. The closed loop bandwidth was nearly 4 Hz, which is capable to track most of the aberration dynamics in a real eye.
    Optics Express 03/2007; 15(4):1946-53. · 3.55 Impact Factor
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    ABSTRACT: In the paper, a method to calculate the time evolution turbulence wavefronts based on the covariance method is theoretically presented in detail. According to it, the time-evolution wavefronts disturbed by atmospheric turbulence were experimentally generated by our LC atmospheric turbulence simulator (ATS) based on liquid crystal on silicon (LCOS) with high pixel density, and measured with a wavefront sensor. The advantage of such a LC ATS over a conventional one is that it is flexible with considering the weather parameters of wind speed and C(n) (2), and is relatively easy to control.
    Optics Express 01/2007; 14(25):11911-8. · 3.55 Impact Factor
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    ABSTRACT: An adaptive optics imaging system is introduced in this paper. A high resolution liquid crystal on silicon (LCOS) device was used as a phase only wave front corrector instead of a conversional deformable mirror. The wave front aberration was detected by a Shack-Hartmann (SH) wave front sensor, which has lambda/100 rms wave front measurement accuracy. Under this construction 0.09lambda (lambda=0.6328microm) Peak to Valley correction precision was reached. Further more, some low frequency hot convection turbulence induced by an electric iron was compensated in real time at the same precision. The Modulation Transfer Function (MTF) of this system was also measured before and after wave front correction. Under the active correction of LCOS, the system reached the diffraction limited resolution approximately 65l p/mm on the horizontal direction. All of this showed the ability of using this device in high resolution, low temporal turbulence imaging system, such as retinal imaging, to improve the resolution performance.
    Optics Express 10/2006; 14(18):8013-8. · 3.55 Impact Factor
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    ABSTRACT: We introduce the novel parallel-aligned liquid crystal wave- front corrector LC WFC with 1920480 pixels designed to operate at phase-only mode. The optical characteristics of the LC WFC were mea- sured. The theory of diffractive optics is used to correct the aberrated wavefront. The measured peak-valve PV value of the wavefront is 9.956 before correction and 0.837 after that. Moreover, the measured root mean square rms value of the wavefront is 2.202 before correc- tion and 0.124 after that. It expands LC devices' application fields.
    Optical Engineering 01/2006; 45(12). · 0.88 Impact Factor
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    ABSTRACT: We illustrate that the phase-only liquid crystal spatial light modulator (LC SLM) can be used for optical testing. The large phase change and the phase modulation precision are discussed. The computer generated holograms (CGH) method is used to acquire the significant phase modulation. And the phase modulating characteristics of the LC SLM are measured. It shows the phase modulation depth is more than 2pi and the modulation precision is down to 1/14lambda (PV) and 1/100lambda (rms) (lambda=632.8nm). In order to verify this method, the former surface of a convex lens is tested by ZYGO interferometer. The parallel straight fringes are obtained. It is shown that PV is 1/3lambda and rms is 1/20lambda after compensated by the LC SLM.
    Optics Express 03/2005; 13(4):1059-65. · 3.55 Impact Factor
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    ABSTRACT: We introduce a novel parallel-aligned liquid-crystal (LC) spatial light modulator (SLM) that has been designed to operate in a phase-only mode for wave-front correction. We measured and analyzed theoretically the electro-optic characteristics of the LC SLM and obtained a peak-to-valley value of 0.07049lambda(lambda= 0.6328microm) after correction. A Strehl ratio of 0.989 indicates the approximate upper limit of an aberrated wave front that the LC SLM can correct when it is used in an adaptive optical system.
    Optics Express 01/2005; 12(26):6403-9. · 3.55 Impact Factor

Publication Stats

147 Citations
49.96 Total Impact Points

Institutions

  • 2005–2014
    • Changchun Institute of Optics, Fine Mechanics and Physics
      Hsin-ching, Jilin Sheng, China
  • 2008–2012
    • Northeast Institute of Geography and Agroecology
      • • Medical Imaging Department
      • • Graduate School
      Peping, Beijing, China
  • 2007
    • Chinese Academy of Sciences
      • Graduate School
      Beijing, Beijing Shi, China