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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 18 — Sep. 4, 2006
  • pp: 8013–8018
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Adaptive optics imaging system based on a high-resolution liquid crystal on silicon device

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


Optics Express, Vol. 14, Issue 18, pp. 8013-8018 (2006)
http://dx.doi.org/10.1364/OE.14.008013


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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 wavefront corrector instead of a conversional deformable mirror. The wavefront aberration was detected by a Shack-Hartmann (SH) wavefront sensor, which has a wavefront measurement accuracy of λ/100 rms (λ = 0.6328 μm). Under this construction, Peak-to-Valley correction precision of 0.09 λ was reached. Furthermore, 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 wavefront 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 possibility of using this device in a high-resolution, low temporal turbulence imaging system, such as retinal imaging, to improve the resolution performance.

© 2006 Optical Society of America

1. Introduction

Most available adaptive optics systems consist of a deformable mirror, which is very expensive and difficult to fabricate. In recent years, people in many countries have started using other devices such as the membrane deformable mirror and liquid crystal spatial light modulator (LC SLM) to replace the conventional deformable mirror and build a lower–cost, efficient adaptive optics system for many applications. It has been demonstrated that LC SLM can provide a convenient and effective means of amplitude or phase modulation [1

1 . D. J. Cho , S. T. Thurman , J. T. Donner , and G. M. Morris , “ Characteristics of a 128×128 liquid crystal spatial light modulator for wave-front generation ,” Opt. Lett. 23 , 969 – 971 ( 1998 ). [CrossRef]

]. Compared with the deformable mirror, the LC SLM has the advantages of low cost, reliability, low power consumption, low price, no moving mechanical components, and high resolution, as mentioned in several papers [2–5

2 . S. R. Restaino , D. M. Payne , J. T. Baker , J. R. Andrews , S. W. Teare , G. C. Gilbreath , D. Dayton , and J. Gonglewski , “ Liquid crystal technology for adaptive optics: un update ,” SPIE 5003 , 187 – 192 ( 2003 ). [CrossRef]

]. However, there are some drawbacks for liquid crystal devices, such as low temporal response, polarization dependence and dispersion. The response time and polarization dependence have been overcome with dual-frequency liquid crystal [6–7

6 . A. Kirby and G. Love , “ Fast, large and controllable phase modulation using dual frequency liquid crystals ,” Opt. Express 12 , 1470 – 1475 ( 2004 ). [CrossRef] [PubMed]

] and quarter wave plate [8–9

8 . G. D. love , “ Liquid crystal phase modulator for unpolarized light ,” Appl. Opt. 32 , 2222 – 2223 ( 1993 ). [CrossRef] [PubMed]

], respectively. Some LC SLM-based adaptive optics systems have had very effective performance in astronomy [10

10 . D. Dayton , J. Gonglewski , S. Restaino , J. Martin , J. Phillips , M. Hartman , P. Kervin , J. Snodgress , S. Browne , N. Heimann , M. Shilko , R. Pohle , B. Carrion , C. Smith , and D. Thiel , “ Demonstration of new technology MEMS and liquid crystal adaptive optics on bright astronomical objects and satellites ,” Opt. Express 10 , 1508 – 1519 ( 2002 ). [PubMed]

], and they exhibit the enormous potential for using liquid crystal devices in wavefront correction.

2. Adaptive optics imaging system

2.1 Experimental setup

The wavefront correction device used in this paper was an LCOS device called LCR2500 from Holoeye Corporation. Its phase stroke at wavelength of 0.6328 μm is 2.2 π without phase wrapping. With phase wrapping, its maximum phase stroke will be up to 200 π according to Ref. [15

15 . 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]

]. A Shack-Hartmann sensor (HASO32, Imagine Optic) was used to measure the wavefront aberration. The detailed parameters for both of them are shown in Table 1.

The lens L1 has three main functions. First, when combined with the light source it is used to simulate an infinity object. The length of the filament was less than 1 mm, and the focus length of L1 was 500 mm, so it is similar to an object with a nearly 0.1° field angle. Second, it is considered as a telescopic lens. Last, L1 and L2 together are used to make the microlens array plane of the HASO32 be conjugated with the LCOS plane and match their pupil size.

Table 1. Detail parameters of LCR2500 and HASO32

table-icon
View This Table
Fig. 1. Schematic diagram of the imaging system: S, light source; P, polarizer; BS, beam splitter; LCOS, liquid crystal on silicon; H, hole; HASO, wavefront sensor; CF, color filter.

2.2 Wavefront correction under low disturbance

Figure 2 shows the photograph of this experiment setup. The red line shows the ray path of light emitted from the tungsten lamp at the right side of the photo. A computer, not shown in this photo, is used to process the aberration signals detected by HASO32 and send the compensation command to the LCOS. The image of filament was recorded by a CCD camera. Because of the obvious dispersion of liquid crystal for white light, the liquid crystal wavefront corrector was able to correct the aberrated wavefront of light only in a narrow wavelength bandwidth range. In addition, the aberration detected by HASO32 was at the wavelength of 0.6328 μm. Therefore, to obtain a clear image, a color filter was inserted before the CCD camera to reject other wavelengths from the image.

Fig. 2. Photograph of the optical layout in lab.
Fig. 3. Images of filament (a) open loop, (b) closed loop, and the wavefront map (c) open loop, (d) closed loop.

2.3 Wavefront correction under dynamic turbulence

In addition, the closed loop correction under dynamic turbulence has been conducted. An electric iron was used to induce hot convection in this optical system, which simulates low temporal frequency turbulence. Here, we only need qualitative analysis of effects due to turbulence. Therefore, turbulence was not quantified in the experiment. The electric iron is located between L1 and the LCOS, as shown in Fig. 4. The convection intensity can be adjusted by changing the distance between the electric iron and the optical axis. The image of filament was blurred and dithered before the loop was closed. When the correction began, it became clear and stable at once. The precision was the same as above. This process can be seen in the movie of Fig. 5.

Fig. 4. Turbulence simulation.
Fig. 5. Closed loop correction under hot convection (1.01MB) [Media 2.

To estimate the performance of this system, the modulation transfer function (MTF) was measured before and after correction. Figure 6 shows the MTF in the horizontal and vertical directions, respectively. This system approximately reached the diffraction limited performance after correction, especially in the 90° direction. The critical frequency increased from approximately 3l p/mm to almost 50l p/mm in the vertical direction and 65l p/mm in the horizontal direction. The resolution characteristic improved dramatically.

Fig. 6. MTF of this imaging system.

3. Conclusion

LCOS devices have mostly been used in large-screen, high-contrast, high-definition, rear projection televisions and personal head-mount displays. Because they have a small pixel pitch and high pixel density, LCOS devices are mostly beneficial for small area, high precision, low temporal frequency, and high-resolution-phase only wavefront correction.

A high-resolution adaptive optics imaging system that is based on LCOS has been introduced in this paper. A Shack-Hartmann (SH) wavefront sensor was used to detect the wavefront aberration. The temporal averaged residual aberration after the closed loop correction was 0.09λ. PV and 0.016λ. rms under dynamic hot convection. The closed loop bandwidth was nearly 4 Hz at 0.8 closed loop gain. This bandwidth was narrow for astronomy but effective for other imaging systems that contain low temporal frequency turbulence, such as retinal imaging. Improving the bandwidth is still a significant task for the future. The MTFs of this demonstration system also have been measured, which showed the effectiveness of using the LCOS device to improve the performance of the imaging system. The critical frequency increased from approximately 3l p/mm to almost 50l p/mm in the vertical direction and 65l p/mm in the horizontal direction. LCOS will be applied to retinal imaging in the future.

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 .

D. J. Cho , S. T. Thurman , J. T. Donner , and G. M. Morris , “ Characteristics of a 128×128 liquid crystal spatial light modulator for wave-front generation ,” Opt. Lett. 23 , 969 – 971 ( 1998 ). [CrossRef]

2 .

S. R. Restaino , D. M. Payne , J. T. Baker , J. R. Andrews , S. W. Teare , G. C. Gilbreath , D. Dayton , and J. Gonglewski , “ Liquid crystal technology for adaptive optics: un update ,” SPIE 5003 , 187 – 192 ( 2003 ). [CrossRef]

3 .

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]

4 .

H. Huang , T. Inoue , and T. Hara , “ An adaptive wavefront control system using a high-resolution liquid crystal spatial light modulator ,” SPIE 5639 , 129 – 137 ( 2004 ). [CrossRef]

5 .

K. A. Bauchert , S. A. Serati , and A. Furman , “ Advances in liquid crystal spatial light modulators ,” SPIE 4734 , 35 – 43 ( 2002 ). [CrossRef]

6 .

A. Kirby and G. Love , “ Fast, large and controllable phase modulation using dual frequency liquid crystals ,” Opt. Express 12 , 1470 – 1475 ( 2004 ). [CrossRef] [PubMed]

7 .

S. R. Restaino , D. Dayton , S. Browne , J. Gonglewski , J. Baker , S. Rogers , S. Mcdermott , J. Gallegos , and M. Shilko , “ On the use of dual frequency nematic material for adaptive optics systems: first results of a closed-loop experiment ,” Opt. Express 6 , 2 – 6 ( 2000 ). [CrossRef] [PubMed]

8 .

G. D. love , “ Liquid crystal phase modulator for unpolarized light ,” Appl. Opt. 32 , 2222 – 2223 ( 1993 ). [CrossRef] [PubMed]

9 .

T. L. Kelly and G. D. Love , “ White-light performance of a polarization-independent liquid crystal phase modulator ,” Appl. Opt. 38 , 1986 – 1989 ( 1999 ). [CrossRef]

10 .

D. Dayton , J. Gonglewski , S. Restaino , J. Martin , J. Phillips , M. Hartman , P. Kervin , J. Snodgress , S. Browne , N. Heimann , M. Shilko , R. Pohle , B. Carrion , C. Smith , and D. Thiel , “ Demonstration of new technology MEMS and liquid crystal adaptive optics on bright astronomical objects and satellites ,” Opt. Express 10 , 1508 – 1519 ( 2002 ). [PubMed]

11 .

Q. Mu , Y. Liu , L. Hu , D. Li , Z. Cao , and L Xuan , “ Determination of anisotropic liquid crystal layer parameters by spectroscopic ellipsometer ,” Acta Phys. Sin. 55 , 1055 – 1060 ( 2006 ).

12 .

Y. Liu , L. Xuan , L. Hu , Z. Cao , D. Li , Q. Mu , and X. Lu , “ Investigation on the liquid crystal spatial light modulator with high precision and pure phase ,” Acta Opt. Sin. 25 , 1682 – 1686 ( 2005 ).

13 .

Y. Liu , L. Hu , Z. Cao , D. Li , Q. Mu , X. Lu , and L. Xuan , “ The investigation of Controllable Phase liquid crystal spatial light modulator ,” Acta Photon. Sin. 34 , 1799 – 1802 ( 2005 ).

14 .

Y. Liu , L. Xuan , L. Hu , Z. Cao , D. Li , Q. Mu , and X. Lu , “ The wavefront modulation characteristics of the parallel aligned liquid crystal device ,” Acta Photon. Sin. 35 , 65 – 68 ( 2006 ).

15 .

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]

16 .

C. Boyer , V. Michau , and G. Rousset , “ Adaptive optics: interaction matrix measurements and real time control algorithms for the COME-ON project ,” SPIE 1237 , 406 – 421 ( 1990 ). [CrossRef]

OCIS Codes
(010.1080) Atmospheric and oceanic optics : Active or adaptive optics
(230.3720) Optical devices : Liquid-crystal devices
(230.6120) Optical devices : Spatial light modulators

ToC Category:
Adaptive Optics

History
Original Manuscript: June 12, 2006
Revised Manuscript: July 24, 2006
Manuscript Accepted: July 27, 2006
Published: September 1, 2006

Virtual Issues
Vol. 1, Iss. 10 Virtual Journal for Biomedical Optics

Citation
Quanquan Mu, Zhaoliang Cao, Lifa Hu, Dayu Li, and Li Xuan, "An adaptive optics imaging system based on a high-resolution liquid crystal on silicon device," Opt. Express 14, 8013-8018 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-18-8013


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References

  1. D. J. Cho, S. T. Thurman, J. T. Donner, and G. M. Morris, "Characteristics of a 128×128 liquid crystal spatial light modulator for wave-front generation," Opt. Lett. 23, 969-971 (1998). [CrossRef]
  2. S. R. Restaino, D. M. Payne, J. T. Baker, J. R. Andrews, S. W. Teare, G. C. Gilbreath, D. Dayton, and J. Gonglewski, "Liquid crystal technology for adaptive optics: un update," in Liquid Crystal Materials, Devices, and Applications IX, L.-C. Chien, ed., Proc. SPIE 5003, 187-192 (2003). [CrossRef]
  3. 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]
  4. H. Huang, T. Inoue, and T. Hara, "An adaptive wavefront control system using a high-resolution liquid crystal spatial light modulator," in Adaptive Optics and Applications III, W. Jiang, and Y. Suzuki, eds., Proc. SPIE 5639, 129-137 (2004). [CrossRef]
  5. K. A. Bauchert, S. A. Serati, and A. Furman, "Advances in liquid crystal spatial light modulators," in Optical Pattern Recognition XIII, D. P. Casasent, T.-H. Chao, eds., Proc. SPIE 4734, 35-43 (2002). [CrossRef]
  6. A. Kirby and G. Love, "Fast large and controllable phase modulation using dual frequency liquid crystals," Opt. Express 12, 1470-1475 (2004). [CrossRef] [PubMed]
  7. S. R. Restaino, D. Dayton, S. Browne, J. Gonglewski, J. Baker, S. Rogers, S. Mcdermott, J. Gallegos, and M. Shilko, "On the use of dual frequency nematic material for adaptive optics systems: first results of a closed-loop experiment," Opt. Express 6, 2-6 (2000). [CrossRef] [PubMed]
  8. G. D. Love, "Liquid crystal phase modulator for unpolarized light," Appl. Opt. 32, 2222-2223 (1993). [CrossRef] [PubMed]
  9. T. L. Kelly and G. D. Love, "White-light performance of a polarization-independent liquid crystal phase modulator," Appl. Opt. 38, 1986-1989 (1999). [CrossRef]
  10. D. Dayton, J. Gonglewski, S. Restaino, J. Martin, J. Phillips, M. Hartman, P. Kervin, J. Snodgress, S. Browne, N. Heimann, M. Shilko, R. Pohle, B. Carrion, C. Smith, and D. Thiel, "Demonstration of new technology MEMS and liquid crystal adaptive optics on bright astronomical objects and satellites," Opt. Express 10, 1508-1519 (2002). [PubMed]
  11. Q. Mu, Y. Liu, L. Hu, D. Li, Z. Cao, and L. Xuan, "Determination of anisotropic liquid crystal layer parameters by spectroscopic ellipsometer," Acta Phys. Sin. 55, 1055-1060 (2006).
  12. Y. Liu, L. Xuan, L. Hu, Z. Cao, D. Li, Q. Mu, and X. Lu, "Investigation on the liquid crystal spatial light modulator with high precision and pure phase," Acta Opt. Sin. 25, 1682-1686 (2005).
  13. Y. Liu, L. Hu, Z. Cao, D. Li, Q. Mu, X. Lu, and L. Xuan, "The investigation of Controllable Phase liquid crystal spatial light modulator," Acta Photon. Sin. 34, 1799-1802 (2005).
  14. Y. Liu, L. Xuan, L. Hu, Z. Cao, D. Li, Q. Mu, and X. Lu, "The wavefront modulation characteristics of the parallel aligned liquid crystal device," Acta Photonica. Sin. 35, 65-68 (2006).
  15. 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]
  16. C. Boyer, V. Michau, and G. Rousset, "Adaptive optics: interaction matrix measurements and real time control algorithms for the COME-ON project," in Amplitude and Intensity Spatial Interferometry, J. B. Breckinridge, ed., Proc. SPIE 1237, 406-421 (1990). [CrossRef]

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