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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 4 — Feb. 19, 2007
  • pp: 1946–1953
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Liquid Crystal based adaptive optics system to compensate both low and high order aberrations in a model eye

Quanquan Mu, Zhaoliang Cao, Dayu Li, Lifa Hu, and Li Xuan  »View Author Affiliations


Optics Express, Vol. 15, Issue 4, pp. 1946-1953 (2007)
http://dx.doi.org/10.1364/OE.15.001946


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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 0.049λ (λ =0.63μm) for over to 10 diopters and the modulation transfer function (MTF) retains 51lp/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.

© 2007 Optical Society of America

1. Introduction

Most kind of adaptive optics retina imaging systems only correct high order aberrations in the human eye to improve the imaging performance. Low order aberrations are often very large and deformable mirror has not such a high stroke. Usually, a four mirror sub system was used to correct the defocus of eye, which makes the whole system complex and could not fully correct these low order aberrations like defocus and astigmatism, which were always large [11

11. E. J. Fernandez, I. Iglesias, and P. Artal, “Closed-loop adaptive optics in the human eye,” Opt. Lett. 26,746748 (2001). [CrossRef]

]. Thibos et al. has reported about the correction of spherical and astigmatic refractive changes in the eye with only about 1.5 diopters with a 127 liquid crystal cells [12

12. L. N. Thibos and A. Bradley, “Use of Liquid-Crystal Adaptive-Optics to alter the refractive state of the eye,” Optom. Vision Sci. 74,581–587 (1997). [CrossRef]

]. It showed the possibility of using liquid crystal device to alter the refractive states in the eye although still too restricted. After that, other liquid crystal based adaptive optics systems still compensate high order aberrations only. Prieto et al. implemented a liquid crystal-based adaptive optics system for close-loop correction of ocular aberrations, discussed its potential and checked its performance in both artificial and real eyes, showed the possibility of using liquid crystal devices as a high-resolution wave-front corrector even in an open-loop configuration [13

13. P. M. Prieto, E. J. Fernandez, S. Manzanera, and P. Artal, “Adaptive optics with a programmable phase modulator: applicaitons in the human eye,” Opt. Express 12,4059–4071 (2004). [CrossRef] [PubMed]

]. Bessho et al. showed some preliminary results on high resolution fundus imaging using a liquid crystal modulator to correct the ocular aberrations [14

14. K. Bessho, T. Yamaguchi, N. Nakazawa, T. Mihashi, Y. Okawaa, N. Maeda, and T. Fujikado, “Live photoreceptor imaging using a prototype adaptive optics fundus camera: A preliminary result,” Invest. Ophthalmol. Vis. Sci. 46,3547 Suppl. S (2005).

]. Fernandez et al. has developed an adaptive optics OCT system which includes a liquid crystal phase modulator and obtained high resolution tomograms of the live retina [15

15. E. J. Fernandez, B. Povazay, B. Hermann, A. Unterhuber, H. Sattmann, P. M. Prieto, R. Leitgeb, P. Ahnelt, P. Artal, and W. Drexler, “Three-dimensional adaptive optics ultrahigh-resolution optical coherence tomography using a liquid crystal spatial light modulator,” Vision Res. 45,3432–3444(2005). [CrossRef] [PubMed]

]. Another Liquid-crystal adaptive optics system based on feedback interferometry was introduced by Shirai who proposed a system for high-resolution retinal imaging using an interferogram-driven liquid crystal modulator [16

16. T. Shirai, “Liquid-crystal adaptive optics based on feedback interferometry for high-resolution retinal imaging,” Appl. Opt. 41,4013–4023(2002). [CrossRef] [PubMed]

]. In this paper, we used a high-resolution LCOS device in a modeled retina imaging adaptive optics system for both low and high order aberrations correction. Based on its high pixel density and small pixel pitch, the phase wrapping technique can be used to expand the modulation depth of the LCOS device. It is suitable for large range low order wave-front error correction besides high order aberrations. This is benefit for retina imaging.

2. Apparatus

Table 1. Table 1. Detail parameters of LCR2500 and HASO32(a) LCR2500

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3. Optical configuration

A simple eye model was build at first as shown in Fig. 1. It consists of a fiber bundle, an achromatic cemented double lens and a diaphragm which represent the retina, the lens and the pupil respectively. The focal length we used here is 60mm, which is approximately twice the focal length of real human eye. The pupil diameter is 2.7mm. The diameter of the fiber bundle is 1mm and the diameter for each fiber is 25 μ m. At this configuration we can get a diffraction limited resolution of nearly 17.16μ m at the focus point of the lens, which is comparable with the diameter of the fiber we used. So it is convenient for us to check whether it reached the diffraction limited resolution or not after the correction. The purpose of this study is to explore the availability of the LCOS device in retina imaging to correct both low and high order aberrations. A 633nm monochromatic light, which was produced by a white light source attached with a 633nm filter, emitted out from the fiber bundle directly to model the retina been illuminated. The light power emitted from the model eye is at a level of 9μ w.

The power density is 157μ w/cm2 which is ten times bigger than the maximum permissible exposure limits for continues exposure for a real eye. The illumination optical system and the safe power for a real eye will be considered in future works. Based on this eye model, we constructed an adaptive optics retinal imaging system uses the LCR2500 to correct the wave-front errors induced by myopia glass and the roughness of the LCOS device.

Fig. 1. Eye model (1. fiber bundle; 2. double lens; 3. diaphragm; 4.myopia glass)

Figure 2 shows the schematic diagram of the adaptive optics retina imaging system. Light emitted out from the eye model was refocused by lens L1 which has the same focal length as the eye model. Then it became horizontally polarized after passing through the polarizer P. The linearly polarized light was collimated by lens L2 before projected onto the LCOS. The L1 and L2 lenses pair also makes sure the LCOS plane is conjugated with the pupil. In the same way, the L2 and L3 conjugated the LCOS to the plane of L4, and then conjugated with the microlens array of the HASO by L5. The CCD camera recorded the retina image at the focus of the lens L4. A hole was used here to reject unwanted light from the system. The actual area on the LCOS we used here was approximately 14mm in diameter based on the zoom effect of L1 and L2. Optical layout in lab is shown in Fig. 3.

Fig. 2. Schematic diagram of the adaptive optics system
Fig. 3. Optical layout in lab

4. Results and discussion

Images of the fiber bundle were recorded before and after wave-front correction. Figure 6 showed the images for different diopters before correction. We can see it blurred and enlarged significantly along with the increase of diopters. After correction, they all became more clear than before. Also we can see that the definition of the image decreased obviously when the diopter larger than 12D. The same result supported by the modulation transfer function (MTF) curve in Fig. 8.

Fig. 4. Wave-front errors of the model eye for different diopters.
Fig. 5. Comparison of the wave-front errors of the model eye between theory and experiment in RMS.
Fig. 6. Images of fiber bundle before wave-front correction for different diopters.
Fig. 7. Images of fiber bundle after wave-front correction for different diopters.

The critical frequency was defined where the MTF value equal to 0.1. Figure 9 showed the critical frequency after correction at different diopters. We can see it changes slightly when the diopter was lower than 10 diopters and it retains a resolution over 51lp/mm, which is nearly the diffraction limited resolution. The residual wave-front error in RMS increased slightly from lower than 0.02λ to nearly 0.049λ for over 10 diopters as shown in Fig. 10. This degradation mainly induced by the misalignment of myopia glass with the whole system and the influence of light rejected by the hole, which may reduce the precision of HASO. Further optimization will be done to improve its performance.

Fig. 8. MTF curve of the whole system for different diopters before and after correction
Fig. 9. The Critical frequency of the whole system for different diopters after correction.
Fig. 10. RMS wave-front error of the model eye before (black) and after (red) correction

5. Conclusion

An adaptive optics retina imaging system has been built based on a simple eye model. A LCOS device was used to correct both low and high order aberrations simulated by myopia glass and the roughness of LCOS respectively. We can get a nearly diffraction limited resolution of 51lp/mm when the wave-front aberration is not more than 10 diopters and the correction precision in RMS remains lower than 0.049λ . All of this demonstrated that liquid crystal devices, which have high pixel density and very small pixel pitch, are very comfortable for large aberration correction. Also there are still a lot of works to improve its performance and used to a real human eye.

Acknowledgments

This work is supported by National Natural Science Foundation (No. 60578035, No. 50473040) and Science Foundation of Jilin Province (No. 20050520, No. 20050321-2).

References and links

1.

H. W. Babcock, “The possibility of compensating astronomical seeing,” Publ. Astron. Soc. Pac. 65,229–236 (1953). [CrossRef]

2.

M. S. Smirnov, “Measurement of wave aberration in the human eye,” Biophysics 6,766–795 (1961).

3.

A. W. Dreher, J. F. Bille, and R. N. Weinreb, “Active optical depth resolution improvement of the laser tomographic scanner,” Appl. Opt. 24,804–808 (1989). [CrossRef]

4.

J. Liang, B. Grimm, S. Goelz, and J. Bille, “Objective measurement of the wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor,” J. Opt. Soc. Am. A 11,1949–1957 (1994). [CrossRef]

5.

J. Liang, D. R. Williams, and D. T. Miller, “Supernormal vision and high-resolution retinal imaging through adaptive optics,” J. Opt. Soc. Am. A 14,2884–2892 (1997). [CrossRef]

6.

J. Carroll, D. C. Gray, A. Roorda, and D. R. Williams, “Recent advances in Retinal Imaging with Adaptive tics,” Opt. Photonics News 16,36–42 (2005). [CrossRef]

7.

Z. Cao, L. Xuan, L. Hu, Y. Liu, and Q. Mu, “Effects of the space-bandwidth product on the liquid-crystal kinoform,” Opt. Express 13,5186–5191 (2005). [CrossRef] [PubMed]

8.

L. Hu, L Xuan, Y. Liu, Z. Cao, D. Li, and Q. Mu, “Phase-only liquid crystal spatial light modulator for wave-front correction with high precision,” Opt. Express 12,6403–6409 (2004). [CrossRef] [PubMed]

9.

F. V. Martin, P. M. Prieto, and P. Artal, “Correction of the aberrations in the human eye with a liquid-crystal spatial light modulator: limits to performance,” J. Opt. Soc. Am. A 15,2552–2562 (1998). [CrossRef]

10.

Q. Mu, Z. Cao, L Hu, D. Li, and L. Xuan, “Adaptive optics imaging system based on a high-resolution liquid crystal on silicon device,” Opt. Express 14,8013–8018 (2006). [CrossRef] [PubMed]

11.

E. J. Fernandez, I. Iglesias, and P. Artal, “Closed-loop adaptive optics in the human eye,” Opt. Lett. 26,746748 (2001). [CrossRef]

12.

L. N. Thibos and A. Bradley, “Use of Liquid-Crystal Adaptive-Optics to alter the refractive state of the eye,” Optom. Vision Sci. 74,581–587 (1997). [CrossRef]

13.

P. M. Prieto, E. J. Fernandez, S. Manzanera, and P. Artal, “Adaptive optics with a programmable phase modulator: applicaitons in the human eye,” Opt. Express 12,4059–4071 (2004). [CrossRef] [PubMed]

14.

K. Bessho, T. Yamaguchi, N. Nakazawa, T. Mihashi, Y. Okawaa, N. Maeda, and T. Fujikado, “Live photoreceptor imaging using a prototype adaptive optics fundus camera: A preliminary result,” Invest. Ophthalmol. Vis. Sci. 46,3547 Suppl. S (2005).

15.

E. J. Fernandez, B. Povazay, B. Hermann, A. Unterhuber, H. Sattmann, P. M. Prieto, R. Leitgeb, P. Ahnelt, P. Artal, and W. Drexler, “Three-dimensional adaptive optics ultrahigh-resolution optical coherence tomography using a liquid crystal spatial light modulator,” Vision Res. 45,3432–3444(2005). [CrossRef] [PubMed]

16.

T. Shirai, “Liquid-crystal adaptive optics based on feedback interferometry for high-resolution retinal imaging,” Appl. Opt. 41,4013–4023(2002). [CrossRef] [PubMed]

17.

Y. Liu, Z. Cao, D. Li, Q. Mu, L. Hu, X. Lu, and L. Xuan, “Correction for large aberration with phase-only liquid-crystal wavefront corrector,” Opt. Eng. 45,128001(2006). [CrossRef]

OCIS Codes
(010.1080) Atmospheric and oceanic optics : Active or adaptive optics
(230.3720) Optical devices : Liquid-crystal devices
(330.4460) Vision, color, and visual optics : Ophthalmic optics and devices

ToC Category:
Vision, Color, and Visual Optics

History
Original Manuscript: November 27, 2006
Revised Manuscript: January 30, 2007
Manuscript Accepted: January 31, 2007
Published: February 19, 2007

Virtual Issues
Vol. 2, Iss. 3 Virtual Journal for Biomedical Optics

Citation
Quanquan Mu, Zhaoliang Cao, Dayu Li, Lifa Hu, and Li Xuan, "Liquid Crystal based adaptive optics system to compensate both low and high order aberrations in a model eye," Opt. Express 15, 1946-1953 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-4-1946


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References

  1. H. W. Babcock, "The possibility of compensating astronomical seeing," Publ. Astron. Soc. Pac. 65, 229-236 (1953). [CrossRef]
  2. M. S. Smirnov, "Measurement of wave aberration in the human eye," Biophysics 6, 766-795 (1961).
  3. A. W. Dreher, J. F. Bille, and R. N. Weinreb, "Active optical depth resolution improvement of the laser tomographic scanner," Appl. Opt. 24, 804-808 (1989). [CrossRef]
  4. J. Liang, B. Grimm, S. Goelz, and J. Bille, "Objective measurement of the wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor," J. Opt. Soc. Am. A 11, 1949-1957 (1994). [CrossRef]
  5. J. Liang, D. R. Williams, and D. T. Miller, "Supernormal vision and high-resolution retinal imaging through adaptive optics," J. Opt. Soc. Am. A 14, 2884-2892 (1997). [CrossRef]
  6. J. Carroll, D. C. Gray, A. Roorda, and D. R. Williams, "Recent advances in Retinal Imaging with Adaptive Optics," Opt. Photonics News 16, 36-42 (2005). [CrossRef]
  7. Z. Cao, L. Xuan, L. Hu, Y. Liu, and Q. Mu, "Effects of the space-bandwidth product on the liquid-crystal kinoform," Opt. Express 13, 5186-5191 (2005). [CrossRef] [PubMed]
  8. L. Hu, L. Xuan, Y. Liu, Z. Cao, D. Li, and Q. Mu, "Phase-only liquid crystal spatial light modulator for wave-front correction with high precision," Opt. Express 12, 6403-6409 (2004). [CrossRef] [PubMed]
  9. F. V. Martin, P. M. Prieto, and P. Artal, "Correction of the aberrations in the human eye with a liquid-crystal spatial light modulator: limits to performance," J. Opt. Soc. Am. A 15, 2552-2562 (1998). [CrossRef]
  10. Q. Mu, Z. Cao, L. Hu, D. Li, and L. Xuan, "Adaptive optics imaging system based on a high-resolution liquid crystal on silicon device," Opt. Express 14, 8013-8018 (2006). [CrossRef] [PubMed]
  11. E. J. Fernandez, I. Iglesias, and P. Artal, "Closed-loop adaptive optics in the human eye," Opt. Lett. 26, 746-748 (2001). [CrossRef]
  12. L. N. Thibos and A. Bradley, "Use of Liquid-Crystal Adaptive-Optics to alter the refractive state of the eye," Optom. Vision Sci. 74, 581-587 (1997). [CrossRef]
  13. P. M. Prieto, E. J. Fernandez, S. Manzanera, and P. Artal, "Adaptive optics with a programmable phase modulator: applicaitons in the human eye," Opt. Express 12, 4059-4071 (2004). [CrossRef] [PubMed]
  14. K. Bessho, T. Yamaguchi, N. Nakazawa, T. Mihashi, Y. Okawaa, N. Maeda, and T. Fujikado, "Live photoreceptor imaging using a prototype adaptive optics fundus camera: A preliminary result," Invest. Ophthalmol. Vis. Sci. 46, 3547 Suppl. S (2005).
  15. E. J. Fernandez, B. Povazay, B. Hermann, A. Unterhuber, H. Sattmann, P. M. Prieto, R. Leitgeb, P. Ahnelt, P. Artal, and W. Drexler, "Three-dimensional adaptive optics ultrahigh-resolution optical coherence tomography using a liquid crystal spatial light modulator," Vision Res. 45, 3432-3444(2005). [CrossRef] [PubMed]
  16. T. Shirai, "Liquid-crystal adaptive optics based on feedback interferometry for high-resolution retinal imaging," Appl. Opt. 41, 4013-4023(2002). [CrossRef] [PubMed]
  17. Y. Liu, Z. Cao, D. Li, Q. Mu, L. Hu, X. Lu, and L. Xuan, "Correction for large aberration with phase-only liquid-crystal wavefront corrector," Opt. Eng. 45, 128001(2006). [CrossRef]

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