## Low speckle laser illuminated projection system with a vibrating diffractive beam shaper |

Optics Express, Vol. 20, Issue 15, pp. 16552-16566 (2012)

http://dx.doi.org/10.1364/OE.20.016552

Acrobat PDF (4170 KB)

### Abstract

Currently the major issues in applying the laser as an illumination source for projectors are beam shaping and laser speckle. We present a compact total solution for both issues by using a diffractive beam shaper associated with a cylindrical lens for the illumination optics and a vibrating motor attached to the beam shaper to eliminate speckle on the projection screen. The diffractive beam shaper features a double-sided microlens array with a lateral shift to each other. The illumination pattern is free of zero diffraction order mainly due to the continuous and spherical surface relief of the lenslet, which can be accurately fabricated with diamond turning and injection molding without quantizing surface relief, so that the illumination pattern on the microdisplay can match the design very well with high diffraction efficiency. In addition, the vibration of the diffractive beam shaper in the longitudinal mode has been found effective for eliminating the dot pattern in the illumination and reducing laser speckle on the projection screen. The proposed laser illuminator has been implemented on a three-panel LCoS projector engine to replace the traditional UHP lamp. The uniformity and speckle contrast are measured to be 78% and 5.5% respectively, which demonstrates the feasibility and potential of the proposed scheme.

© 2012 OSA

## 1. Introduction

1. C. M. Chang and H. P. D. Shieh, “Design of illumination and projection optics for projectors with single digital micromirror devices,” Appl. Opt. **39**(19), 3202–3208 (2000). [CrossRef] [PubMed]

3. J. W. Pan, C. M. Wang, H. C. Lan, W. S. Sun, and J. Y. Chang, “Homogenized LED-illumination using microlens arrays for a pocket-sized projector,” Opt. Express **15**(17), 10483–10491 (2007). [CrossRef] [PubMed]

4. P. C. Chen, C. C. Chen, P. H. Yao, and C. H. Chen, “Double side lenslet array for illumination optics of laser projector,” Proc. SPIE **7232**, 72320X, 72320X-9 (2009). [CrossRef]

5. S. Zhang, “A simple bi-convex refractive laser beam shaper,” J. Opt. A, Pure Appl. Opt. **9**(10), 945–950 (2007). [CrossRef]

6. F. Wippermann, U.-D. Zeitner, P. Dannberg, A. Bräuer, and S. Sinzinger, “Beam homogenizers based on chirped microlens arrays,” Opt. Express **15**(10), 6218–6231 (2007). [CrossRef] [PubMed]

7. C. Dorrer and J. D. Zuegel, “Design and analysis of binary beam shapers using error diffusion,” J. Opt. Soc. Am. B **24**(6), 1268–1275 (2007). [CrossRef]

8. R. M. Tasso, Sales, Geoffrey Gretton, G. Michael Morris, and Daniel H. Raguin, “Beam shaping and homogenization with random microlens arrays,” in *Diffractive Optics and Micro-Optics*, R. Magnusson, ed., Vol. 75 of OSA Trends in Optics and Photonics Series (Optical Society of America, 2002), paper DMA3.

6. F. Wippermann, U.-D. Zeitner, P. Dannberg, A. Bräuer, and S. Sinzinger, “Beam homogenizers based on chirped microlens arrays,” Opt. Express **15**(10), 6218–6231 (2007). [CrossRef] [PubMed]

8. R. M. Tasso, Sales, Geoffrey Gretton, G. Michael Morris, and Daniel H. Raguin, “Beam shaping and homogenization with random microlens arrays,” in *Diffractive Optics and Micro-Optics*, R. Magnusson, ed., Vol. 75 of OSA Trends in Optics and Photonics Series (Optical Society of America, 2002), paper DMA3.

10. J. W. Goodman, “Some fundamental properties of speckle,” J. Opt. Soc. Am. **66**(11), 1145–1150 (1976). [CrossRef]

11. A. Furukawa, N. Ohse, Y. Sato, D. Imanishi, K. Wakabayashi, S. Ito, K. Tamamura, and S. Hirata, “Effective speckle reduction in laser projection displays,” Proc. SPIE **6911**, 69110T, 69110T-7 (2008). [CrossRef]

12. G. M. J. Craggs, F. Riechert, Y. Meuret, H. Thienpont, U. Lemmer, and G. Verschaffelt, “Low-speckle laser projection using farfield nonmodal emission of a broad-area vertical-cavity surface-emitting laser,” Proc. SPIE **7720**, 772020, 772020-8 (2010). [CrossRef]

13. M. N. Akram, V. Kartashov, and Z. M. Tong, “Speckle reduction in line-scan laser projectors using binary phase codes,” Opt. Lett. **35**(3), 444–446 (2010). [CrossRef] [PubMed]

14. T. Iwai and T. Asakura, “Speckle reduction in coherent information processing,” Proc. IEEE **84**(5), 765–781 (1996). [CrossRef]

15. L. L. Wang, T. Tschudi, T. Halldórsson, and P. R. Pétursson, “Speckle reduction in laser projection systems by diffractive optical elements,” Appl. Opt. **37**(10), 1770–1775 (1998). [CrossRef] [PubMed]

19. G. Ouyang, Z. M. Tong, M. N. Akram, K. V. Wang, V. Kartashov, X. Yan, and X. Y. Chen, “Speckle reduction using a motionless diffractive optical element,” Opt. Lett. **35**(17), 2852–2854 (2010). [CrossRef] [PubMed]

15. L. L. Wang, T. Tschudi, T. Halldórsson, and P. R. Pétursson, “Speckle reduction in laser projection systems by diffractive optical elements,” Appl. Opt. **37**(10), 1770–1775 (1998). [CrossRef] [PubMed]

16. L. L. Wang, T. Tschudi, M. Boeddinghaus, A. Elbert, T. Halldorsson, and P. Petursson, “Speckle reduction in laser projections with ultrasonic waves,” Opt. Eng. **39**(6), 1659–1664 (2000). [CrossRef]

17. M. N. Akram, Z. M. Tong, G. M. Ouyang, X. Y. Chen, and V. Kartashov, “Laser speckle reduction due to spatial and angular diversity introduced by fast scanning micromirror,” Appl. Opt. **49**(17), 3297–3304 (2010). [CrossRef] [PubMed]

18. E. G. Rawson, A. B. Nafarrate, R. E. Norton, and J. W. Goodman, “Speckle-free rear-projection screen using two close screens in slow relative motion,” J. Opt. Soc. Am. **66**(11), 1290–1294 (1976). [CrossRef]

19. G. Ouyang, Z. M. Tong, M. N. Akram, K. V. Wang, V. Kartashov, X. Yan, and X. Y. Chen, “Speckle reduction using a motionless diffractive optical element,” Opt. Lett. **35**(17), 2852–2854 (2010). [CrossRef] [PubMed]

20. L. Golan and S. Shoham, “Speckle elimination using shift-averaging in high-rate holographic projection,” Opt. Express **17**(3), 1330–1339 (2009). [CrossRef] [PubMed]

## 2. Diffractive laser beam shaper for illumination optics

4. P. C. Chen, C. C. Chen, P. H. Yao, and C. H. Chen, “Double side lenslet array for illumination optics of laser projector,” Proc. SPIE **7232**, 72320X, 72320X-9 (2009). [CrossRef]

4. P. C. Chen, C. C. Chen, P. H. Yao, and C. H. Chen, “Double side lenslet array for illumination optics of laser projector,” Proc. SPIE **7232**, 72320X, 72320X-9 (2009). [CrossRef]

*S*. The phase distribution function

_{L}*ψ(ζ,η)*provided by the microlens array needs to turn the circular Gaussian beam

*U*into a rectangular top-hat distribution

_{i}*U*on the microdisplay panel at the specified distance

_{o}(x,y;z)*z*. According to scalar diffraction theory,

_{0}*U*is the Fresnel diffraction pattern of

_{o}(x,y;z)*U*, which is the complex field distribution at the exit of microlens array. Equation (1) expresses the mathematical representation of the relationship.where

_{m}(ζ,η)*U*and

_{m}(ζ,η) = U_{i}(ζ,η)e^{iψ(ζ,η)}*h(x,y) =*(

*e*)

^{jkz}/jλz*exp*[

*jk(x*] is the convolution kernel; Σ denotes the illuminated aperture of the beam shaper.

^{2}+ y^{2})/2z*ψ(ζ,η)*are the pitch and the curvature of the lenslet, and the lateral shift

*S*. Distance

_{L}*z*can also be used as a parameter with limited range, which is the adjustable range of the distance between the beam shaper and microdisplay. On the other hand, the criteria for optimizing

_{0}*U*on the microdisplay side are the area of diffraction pattern which should match the active area of microdisplay and a collection efficiency above 80% in the active area. Those design parameters couple together and iteration process employing fast Fourier transform (FFT) algorithm is required before all the criteria can be met. Once the phase distribution function for the desired

_{o}(x,y;z)*U*is found, the thickness distribution of the beam shaper can be obtained from Eq. (2).where

_{o}(x,y;z)*T(ζ,η)*denotes the periodic thickness function of the beam shaper,

*k*denotes the wave number and is refraction index of the beam shaper.

_{0}is calculated to be 164mm considering the refractive index of all the glass material in the color separation unit. The design process shows difficulty to obtain a uniform diffractive pattern with rectangular shape lenslets, and the result came out with square lenslets having the radius of curvature of 325.8μm and the pitch of 140μm respectively. The lateral shift between two side microlens arrays became 90.5μm. Figure 4 shows the simulated field distribution at the distance

*z*= 164mm (target) and 5m respectively. It shows that the dot pitch increases with the propagation distance, and the uniformity degrades in the mean time. The optimization process has ensured that the dot pitch is less than 10μm, roughly the size of the pixel, and there is no dramatic change of local uniformity in the whole pattern. The aspect ratio of the illumination pattern will be reshaped to match the microdisplay with an attached cylindrical lens.

_{0}^{2}. The pattern is examined at the distance z

_{0}= 25mm and z

_{0}= 164mm. Table 2 shows the result and it indicates that the uniformity maintains quite well but the pitch of the dot array increases with a distance. Being a diffractive element performing strong diffraction effects, the most notable feature is no zero-order diffraction exists and the dispersion effect is negligible as shown in Fig. 9 . It indicates that the difference of FWHM among the red, green and blue illumination patterns is lower than 5%. The weak dispersion phenomenon can be considered as a minor factor affecting illumination efficiency in the projection system. The test result also implies that the diffractive beam shaper designed for z

_{0}= 164mm can still be useful for smaller panel size with shorter illumination light path.

## 3. Dot pattern elimination and laser speckle reduction with vibrating beam shaper

### 3.1 Illumination quality analysis on the microdisplay with the vibrating beam shaper

*f*), and the result is shown in Fig. 10 . Above input current of 30mA, the amplitude starts to become steady at 0.3mm c.a.

### 3.2 Evaluation of speckle reduction on the projection screen

_{s}denotes the standard deviation of intensity

*I*over the measurement region and is the average intensity.

_{L}. It indicates that C

_{L}drops significantly with the increasing of driving current below 30mA, where vibration amplitude increases largely with the increasing driving current. There becomes no significant change of C

_{L}above 30mA where vibration amplitude becomes steady with the increase of driving current, although the vibrating frequency still keeps increasing.

_{T}. It indicates that C

_{T}shows similar trend as C

_{L}in corresponding to the change of driving current. However, the effectiveness of the speckle reduction is much less than the case of the longitudinal vibration mode. The achievable lowest contrast is 22%.

## 4. Conclusions

## Acknowledgments

## References and links

1. | C. M. Chang and H. P. D. Shieh, “Design of illumination and projection optics for projectors with single digital micromirror devices,” Appl. Opt. |

2. | X. Zhao, Z. L. Fang, J. C. Cui, X. Zhang, and G. G. Mu, “Illumination system using LED sources for pocket-size projectors,” Appl. Opt. |

3. | J. W. Pan, C. M. Wang, H. C. Lan, W. S. Sun, and J. Y. Chang, “Homogenized LED-illumination using microlens arrays for a pocket-sized projector,” Opt. Express |

4. | P. C. Chen, C. C. Chen, P. H. Yao, and C. H. Chen, “Double side lenslet array for illumination optics of laser projector,” Proc. SPIE |

5. | S. Zhang, “A simple bi-convex refractive laser beam shaper,” J. Opt. A, Pure Appl. Opt. |

6. | F. Wippermann, U.-D. Zeitner, P. Dannberg, A. Bräuer, and S. Sinzinger, “Beam homogenizers based on chirped microlens arrays,” Opt. Express |

7. | C. Dorrer and J. D. Zuegel, “Design and analysis of binary beam shapers using error diffusion,” J. Opt. Soc. Am. B |

8. | R. M. Tasso, Sales, Geoffrey Gretton, G. Michael Morris, and Daniel H. Raguin, “Beam shaping and homogenization with random microlens arrays,” in |

9. | J. W. Goodman, |

10. | J. W. Goodman, “Some fundamental properties of speckle,” J. Opt. Soc. Am. |

11. | A. Furukawa, N. Ohse, Y. Sato, D. Imanishi, K. Wakabayashi, S. Ito, K. Tamamura, and S. Hirata, “Effective speckle reduction in laser projection displays,” Proc. SPIE |

12. | G. M. J. Craggs, F. Riechert, Y. Meuret, H. Thienpont, U. Lemmer, and G. Verschaffelt, “Low-speckle laser projection using farfield nonmodal emission of a broad-area vertical-cavity surface-emitting laser,” Proc. SPIE |

13. | M. N. Akram, V. Kartashov, and Z. M. Tong, “Speckle reduction in line-scan laser projectors using binary phase codes,” Opt. Lett. |

14. | T. Iwai and T. Asakura, “Speckle reduction in coherent information processing,” Proc. IEEE |

15. | L. L. Wang, T. Tschudi, T. Halldórsson, and P. R. Pétursson, “Speckle reduction in laser projection systems by diffractive optical elements,” Appl. Opt. |

16. | L. L. Wang, T. Tschudi, M. Boeddinghaus, A. Elbert, T. Halldorsson, and P. Petursson, “Speckle reduction in laser projections with ultrasonic waves,” Opt. Eng. |

17. | M. N. Akram, Z. M. Tong, G. M. Ouyang, X. Y. Chen, and V. Kartashov, “Laser speckle reduction due to spatial and angular diversity introduced by fast scanning micromirror,” Appl. Opt. |

18. | E. G. Rawson, A. B. Nafarrate, R. E. Norton, and J. W. Goodman, “Speckle-free rear-projection screen using two close screens in slow relative motion,” J. Opt. Soc. Am. |

19. | G. Ouyang, Z. M. Tong, M. N. Akram, K. V. Wang, V. Kartashov, X. Yan, and X. Y. Chen, “Speckle reduction using a motionless diffractive optical element,” Opt. Lett. |

20. | L. Golan and S. Shoham, “Speckle elimination using shift-averaging in high-rate holographic projection,” Opt. Express |

**OCIS Codes**

(110.6150) Imaging systems : Speckle imaging

(120.2040) Instrumentation, measurement, and metrology : Displays

(220.4000) Optical design and fabrication : Microstructure fabrication

(110.2945) Imaging systems : Illumination design

**ToC Category:**

Imaging Systems

**History**

Original Manuscript: May 23, 2012

Revised Manuscript: June 25, 2012

Manuscript Accepted: June 25, 2012

Published: July 6, 2012

**Citation**

Po-Hung Yao, Chieh-Hui Chen, and Cheng-Huan Chen, "Low speckle laser illuminated projection system with a vibrating diffractive beam shaper," Opt. Express **20**, 16552-16566 (2012)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-15-16552

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### References

- C. M. Chang and H. P. D. Shieh, “Design of illumination and projection optics for projectors with single digital micromirror devices,” Appl. Opt.39(19), 3202–3208 (2000). [CrossRef] [PubMed]
- X. Zhao, Z. L. Fang, J. C. Cui, X. Zhang, and G. G. Mu, “Illumination system using LED sources for pocket-size projectors,” Appl. Opt.46(4), 522–526 (2007). [CrossRef] [PubMed]
- J. W. Pan, C. M. Wang, H. C. Lan, W. S. Sun, and J. Y. Chang, “Homogenized LED-illumination using microlens arrays for a pocket-sized projector,” Opt. Express15(17), 10483–10491 (2007). [CrossRef] [PubMed]
- P. C. Chen, C. C. Chen, P. H. Yao, and C. H. Chen, “Double side lenslet array for illumination optics of laser projector,” Proc. SPIE7232, 72320X, 72320X-9 (2009). [CrossRef]
- S. Zhang, “A simple bi-convex refractive laser beam shaper,” J. Opt. A, Pure Appl. Opt.9(10), 945–950 (2007). [CrossRef]
- F. Wippermann, U.-D. Zeitner, P. Dannberg, A. Bräuer, and S. Sinzinger, “Beam homogenizers based on chirped microlens arrays,” Opt. Express15(10), 6218–6231 (2007). [CrossRef] [PubMed]
- C. Dorrer and J. D. Zuegel, “Design and analysis of binary beam shapers using error diffusion,” J. Opt. Soc. Am. B24(6), 1268–1275 (2007). [CrossRef]
- R. M. Tasso, Sales, Geoffrey Gretton, G. Michael Morris, and Daniel H. Raguin, “Beam shaping and homogenization with random microlens arrays,” in Diffractive Optics and Micro-Optics, R. Magnusson, ed., Vol. 75 of OSA Trends in Optics and Photonics Series (Optical Society of America, 2002), paper DMA3.
- J. W. Goodman, Speckle Phenomena in Optics: Theory and Applications (Roberts & Company, 2006).
- J. W. Goodman, “Some fundamental properties of speckle,” J. Opt. Soc. Am.66(11), 1145–1150 (1976). [CrossRef]
- A. Furukawa, N. Ohse, Y. Sato, D. Imanishi, K. Wakabayashi, S. Ito, K. Tamamura, and S. Hirata, “Effective speckle reduction in laser projection displays,” Proc. SPIE6911, 69110T, 69110T-7 (2008). [CrossRef]
- G. M. J. Craggs, F. Riechert, Y. Meuret, H. Thienpont, U. Lemmer, and G. Verschaffelt, “Low-speckle laser projection using farfield nonmodal emission of a broad-area vertical-cavity surface-emitting laser,” Proc. SPIE7720, 772020, 772020-8 (2010). [CrossRef]
- M. N. Akram, V. Kartashov, and Z. M. Tong, “Speckle reduction in line-scan laser projectors using binary phase codes,” Opt. Lett.35(3), 444–446 (2010). [CrossRef] [PubMed]
- T. Iwai and T. Asakura, “Speckle reduction in coherent information processing,” Proc. IEEE84(5), 765–781 (1996). [CrossRef]
- L. L. Wang, T. Tschudi, T. Halldórsson, and P. R. Pétursson, “Speckle reduction in laser projection systems by diffractive optical elements,” Appl. Opt.37(10), 1770–1775 (1998). [CrossRef] [PubMed]
- L. L. Wang, T. Tschudi, M. Boeddinghaus, A. Elbert, T. Halldorsson, and P. Petursson, “Speckle reduction in laser projections with ultrasonic waves,” Opt. Eng.39(6), 1659–1664 (2000). [CrossRef]
- M. N. Akram, Z. M. Tong, G. M. Ouyang, X. Y. Chen, and V. Kartashov, “Laser speckle reduction due to spatial and angular diversity introduced by fast scanning micromirror,” Appl. Opt.49(17), 3297–3304 (2010). [CrossRef] [PubMed]
- E. G. Rawson, A. B. Nafarrate, R. E. Norton, and J. W. Goodman, “Speckle-free rear-projection screen using two close screens in slow relative motion,” J. Opt. Soc. Am.66(11), 1290–1294 (1976). [CrossRef]
- G. Ouyang, Z. M. Tong, M. N. Akram, K. V. Wang, V. Kartashov, X. Yan, and X. Y. Chen, “Speckle reduction using a motionless diffractive optical element,” Opt. Lett.35(17), 2852–2854 (2010). [CrossRef] [PubMed]
- L. Golan and S. Shoham, “Speckle elimination using shift-averaging in high-rate holographic projection,” Opt. Express17(3), 1330–1339 (2009). [CrossRef] [PubMed]

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