## Redistribution of output weighting coefficients for complex multiplexed phase-diffractive elements

Optics Express, Vol. 12, Issue 19, pp. 4347-4352 (2004)

http://dx.doi.org/10.1364/OPEX.12.004347

Acrobat PDF (146 KB)

### Abstract

The formation of multiplexed phase-only holograms with more weighted phase functions creates spurious cross terms and nonlinear scaling. We extend previously reported work [Appl. Opt. **25**, 3767 (1986)] by proposing a normal method to analyze multiplexed holograms mathematically. We show that the output of holograms with any number weighted phase function can be written as a new linear combination for the original phase function with new weights. The relationship between the original weights and the new weights is developed for real-time optimization of hologram performance. We focus on the analysis of two and three multiplexed holograms to demonstrate the effectiveness of this approach.

© 2004 Optical Society of America

## 1. Introduction

1. D. P. Casasent, “Unified synthetic discriminant function computational formulation,” Appl. Opt. **23**, 1620–1627 (1984). [CrossRef] [PubMed]

2. J. A. Davis, S. W. Connely, G. W. Bach, R. A. Lilly, and D. M. Cottrell, “Programmable optical interconnections with large fun-out capability using the magneto-optic spatial light modulator,” Opt. Lett. **14**, 102–104 (1989). [CrossRef] [PubMed]

3. J. A. Davis, D. M. Cottrell, R. A. Lilly, and S. W. Connely, “Multiplexed phase-encoded lenses written on spatial light modulator,” Opt. Lett. **14**, 420–422 (1989). [CrossRef] [PubMed]

4. S. Reichelt and H. J. Tiziani, “Twin-CGHs for absolute calibration in wavefront testing interferometry,” Opt. Commun. **220**, 23–32 (2003). [CrossRef]

5. M. Beyerlein, N. Lindlein, and J. Schwider, “Dual-wave-front computer-generated holograms for quasi-absolute testing of aspherics,” Appl. Opt. **41**, 2440–2447 (2002). [CrossRef] [PubMed]

6. J. A. Davis, E. A. Merrill, D. M. Cottrell, and R. M. Bunch, “Effects of sampling and binarization in the output of the joint Fourier transform correlator,” Opt. Eng. **29**, 1094–1100 (1990). [CrossRef]

7. J. L. Horner and P. D. Gianino, “Applying the phase-only filter concept to the synthetic discriminant function correlation filter,” Appl. Opt. **24**, 851–855 (1985). [CrossRef] [PubMed]

10. E. Carcole, M. S. Millan, and J. Campos, “Derivation of weighting coefficients for multiplexed phase-diffractive elements,” Opt. Lett. **20**, 2360–2362 (1995). [CrossRef] [PubMed]

*N*-weighted phase functions.

8. D. P. Casasent and W. A. Rozzi, “Computer-generated and phase-only synthetic discriminant function filters,” Appl. Opt. **25**, 3767–3772 (1986). [CrossRef] [PubMed]

*N*-weighted phase functions. We show that the output can be written as a new linear combination for the original phase functions with new weights. The relationship between the original weights and the new weights is developed for real-time optimization of hologram performance. We use a fast and efficient way to fit polynomials to obtain the weights for multiplexed holograms that correspond to the desired output and efficiency. In particular we analyze a trifocal hologram with our method. In addition, a bifocal hologram can be used in lieu of a trifocal hologram under some conditions, and the results obtained with the trifocal hologram are identical with those reported in Ref. 8

8. D. P. Casasent and W. A. Rozzi, “Computer-generated and phase-only synthetic discriminant function filters,” Appl. Opt. **25**, 3767–3772 (1986). [CrossRef] [PubMed]

*N*phase functions can be used without the need for analysis when we analyze a hologram with

*N*-weighted phase functions.

## 2. Theory

*N*phase functions exp(

*iϕ*

_{n}) (

*ϕ*

_{n}, where

*n*= 1,2…

*N*are two-dimensional functions) with real weights

*A*

_{n}we assume that

*M*exp(

*ia*), where

*M*is the amplitude and

*a*is the phase:

*M*> 0, a key point in our development is that 1/

*M*can be considered a periodic function of (

*ϕ*

_{1}-

*ϕ*

_{2}), (

*ϕ*

_{1}-

*ϕ*

_{3})⋯(

*ϕ*

_{1}-

*ϕ*

_{N})with period 2

*π*. This allows a Fourier series expansion to be performed:

*m*

_{1},

*m*

_{2}⋯

*m*

_{n-1}are integral numbers, and weights

*a*

_{m1m2⋯mN-1}can be written as

*a*

_{m1m2⋯mN-1}can be obtained. First, weights

*a*

_{m1m2⋯mN-1}are real, because, from Eq. (3),

*M*′(

*β*

_{1},

*β*

_{2}⋯

*β*

_{N-1})=

*M*′(-

*β*

_{1},-

*β*

_{2}⋯-

*β*

_{N-1}). Second, from Eqs. (3) and (5)

*a*

_{m1m2⋯mN-1}depends only on

*A*

_{n}and

*n*and is completely independent of

*ϕ*

_{n}. Inserting Eq. (4) into Eq. (2) and using the definition of

*β*

_{1}⋯

*β*

_{N-1}1 given in Eq. (4) finally yield

*N*-multiplexed phase functions can be written as a new linear combination of the original functions affected by the new weights

*a*

_{n}. Second, the phase-only operation can introduce spurious cross terms. Since the new weights of the output depend only on the original weight of the input, one can perform an excellent polynomial fit on their dependence.

## 3. Validation

*N*= 3, according to Eq. (6) the phase-only diffractive element that corresponds to three multiplexed phase functions can be written as

*x*

_{1}= (

*a*

^{2}/

*a*

^{1})

^{2}and

*x*

_{2}= (

*a*

^{3}/

*a*

^{1})

^{2}, the input ratios as

*y*

^{1}= (

*A*

_{2}/

*A*

_{1})

^{2}and

*y*

^{2}= (

*A*

_{3}/

*A*

_{1})

^{2}. Then the relationship between the input ratios and the output ratios shown in Fig. 1 can be obtained quickly and efficiently by use of a fast Fourier transform (FFT) operation. The diffraction efficiency, defined as

*k*is the degree of these polynomials. The polynomial coefficients can be determined by performing a least-squares fit of the polynomial on the dependence of input and output ratios. Here we chose

*k*= 7 and the polynomials are

*y*

_{1}and

*y*

_{2}is less than 0.002. We could achieve higher accuracy by increasing the degree of the polynomial and the sampling number. Now we can control the weights of the output in real time by adjusting the weights of the input.

*y*

_{1}= 0 (results in

*x*

_{1}= 0) or

*y*

_{2}= 0 (results in

*x*

_{2}= 0). So the relationship between the input ratio and the output ratio of the two multiplexed holograms shown in Fig. 3 can be obtained quickly from Fig. 1 [with

*x*

_{2}= 0 in Fig. 1(a) or with

*x*

_{1}= 0 in Fig. 1(b)]. The diffraction efficiency shown in Fig. 3(b) can be obtained from Fig. 2 with

*x*

_{1}= 0 or

*x*

_{2}= 0. It is clear that the numerical results are the same as those reported in Ref. [8

8. D. P. Casasent and W. A. Rozzi, “Computer-generated and phase-only synthetic discriminant function filters,” Appl. Opt. **25**, 3767–3772 (1986). [CrossRef] [PubMed]

## 4. Simulation

*N*= 3 by quantifying the two-dimensional phase function with our computer program. Many possibilities can be considered. We consider the particular case of three Fresnel lens with the same focal length

*f*one equivalent to a center lens and each of the other two shifted from the origin by an amount

*a*and

*b*in the

*x*direction. If

*x*

_{1}= 0.5 and

*x*

_{2}= 0.6, with Eqs. (9) and (10) we obtain

*y*

_{1}= 0.5383 and

*y*

_{2}= 0.6564. This implies that

*A*

_{1}= 0.7623,

*A*

_{2}= 0.4038, and

*A*

_{3}= 0.5004. In such a case

*ϕ*

_{1}= (

*k*/2

*f*)(

*x*

^{2}+

*y*

^{2}),

*ϕ*

_{2}= (

*k*/2

*f*)⌊(

*x*-

*a*)

^{2}+

*y*

^{2}⌋ , and

*ϕ*

_{3}= (

*k*/2

*f*)⌊(

*x*+

*b*)

^{2}+

*y*

^{2}⌋, where

*k*is the wave number. Substituting the phase functions above into Eq. (6), we can write the resulting trifocal lens as

*m*

_{1}and

*m*

_{2}such as

*m*

_{1}= 0,

*m*

_{2}= 0;

*m*

_{1}= 1,

*m*

_{2}= 0;

*m*

_{1}= 0,

*m*

_{2}= 1;

*m*

_{1}= -1,

*m*

_{2}= 0;

*m*

_{1}= 0,

*m*

_{2}= -1;

*m*

_{1}= -1,

*m*

_{2}= 1;

*m*

_{1}= 1,

*m*

_{2}= -1, into Eq. (11), we obtain the desired output orders that posit at

*x*= -

*b*,

*x*= 0 and

*x*= +

*a*as shown in Eq. (7). At the same time, we also obtain the spurious adjustments that appear at positions

*x*= (

*m*

_{1}

*a*-

*m*

_{2}

*b*),

*x*= (

*m*

_{1}

*a*-

*m*

_{2}

*b*-

*a*) or

*x*= (

*m*

_{1}

*a*-

*m*

_{2}

*b*+

*b*). Note that some spurious orders could coincide with the desired Fresnel lenses but with different phase functions. This could create a difference between the real weight of the output and the calculated weights. However, the problem can be compensated by taking these unwanted orders into account, when we calculate the relationship between the input ratios and the output ratios according to Eqs. (5) and (7).

*a*= 450 μm,

*b*= 525 μm, and

*f*= 1.138 m for a wavelength of 632.8 nm (He-Ne laser). The reconstruction of the hologram calculated by a Fourier transform of the hologram is shown in Fig. 4(b), in which the results are in agreement with the expected ratios. In addition, the locations of the peaks are at

*x*= -525 μm,

*x*= 0, and

*x*= 450 μm, in agreement with Eq. (11). Some spurious orders appear at positions

*x*= (

*m*

_{1}

*a*-

*m*

_{2}

*b*),

*x*= (

*m*

_{1}

*a*-

*m*

_{2}

*b*-

*a*), or

*x*= (

*m*

_{1}

*a*-

*m*

_{2}

*b*+

*b*).

## 5. Conclusion

*N*-weighted phase functions has been studied mathematically. We have shown that the final diffractive element contains a linear combination of the original phase functions affected by new weights and that some undesirable additional terms were created. These new weights depend only on the original weights of the input. Polynomial fitting is used to control the performance of the holograms quickly and efficiently. As an example, we studied a phase-diffractive element constructed from a linear combination of three weighted phase functions and the codification of three multiplexed Fresnel lenses to discover their performance characteristics. This method is suitable for any complex multiplexed phase-diffractive element except for

*M*= 0; however,

*M*= 0 can be calculated by use of the polynomial fitting reported in this paper.

## Acknowledgments

## References and links

1. | D. P. Casasent, “Unified synthetic discriminant function computational formulation,” Appl. Opt. |

2. | J. A. Davis, S. W. Connely, G. W. Bach, R. A. Lilly, and D. M. Cottrell, “Programmable optical interconnections with large fun-out capability using the magneto-optic spatial light modulator,” Opt. Lett. |

3. | J. A. Davis, D. M. Cottrell, R. A. Lilly, and S. W. Connely, “Multiplexed phase-encoded lenses written on spatial light modulator,” Opt. Lett. |

4. | S. Reichelt and H. J. Tiziani, “Twin-CGHs for absolute calibration in wavefront testing interferometry,” Opt. Commun. |

5. | M. Beyerlein, N. Lindlein, and J. Schwider, “Dual-wave-front computer-generated holograms for quasi-absolute testing of aspherics,” Appl. Opt. |

6. | J. A. Davis, E. A. Merrill, D. M. Cottrell, and R. M. Bunch, “Effects of sampling and binarization in the output of the joint Fourier transform correlator,” Opt. Eng. |

7. | J. L. Horner and P. D. Gianino, “Applying the phase-only filter concept to the synthetic discriminant function correlation filter,” Appl. Opt. |

8. | D. P. Casasent and W. A. Rozzi, “Computer-generated and phase-only synthetic discriminant function filters,” Appl. Opt. |

9. | R. R Kallman, “Optimal low noise phase-only and binary phase-only optical correlation filters for threshold detectors,” Appl. Opt. |

10. | E. Carcole, M. S. Millan, and J. Campos, “Derivation of weighting coefficients for multiplexed phase-diffractive elements,” Opt. Lett. |

**OCIS Codes**

(090.4220) Holography : Multiplex holography

(230.4110) Optical devices : Modulators

**ToC Category:**

Research Papers

**History**

Original Manuscript: August 12, 2004

Revised Manuscript: August 26, 2004

Published: September 20, 2004

**Citation**

Hua Liu, Zhenwu Lu, and Fengyou Li, "Redistribution of output weighting coefficients for complex multiplexed phase-diffractive elements," Opt. Express **12**, 4347-4352 (2004)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-19-4347

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### References

- D. P. Casasent, �??Unified synthetic discriminant function computational formulation,�?? Appl. Opt. 23, 1620�??1627 (1984). [CrossRef] [PubMed]
- J. A. Davis, S. W. Connely, G. W. Bach, R. A. Lilly, and D. M. Cottrell, �??Programmable optical interconnections with large fun-out capability using the magneto-optic spatial light modulator,�?? Opt. Lett. 14, 102�??104 (1989). [CrossRef] [PubMed]
- J. A. Davis, D. M. Cottrell, R. A. Lilly, and S. W. Connely, �??Multiplexed phase-encoded lenses written on spatial light modulator,�?? Opt. Lett. 14, 420�??422 (1989). [CrossRef] [PubMed]
- S. Reichelt and H. J. Tiziani, �??Twin-CGHs for absolute calibration in wavefront testing interferometry,�?? Opt. Commun. 220, 23�??32 (2003). [CrossRef]
- M. Beyerlein, N. Lindlein, and J. Schwider, �??Dual-wave-front computer-generated holograms for quasi-absolute testing of aspherics,�?? Appl. Opt. 41, 2440�??2447 (2002). [CrossRef] [PubMed]
- J. A. Davis, E. A. Merrill, D. M. Cottrell, and R. M. Bunch, �??Effects of sampling and binarization in the output of the joint Fourier transform correlator,�?? Opt. Eng. 29, 1094�??1100 (1990). [CrossRef]
- J. L. Horner and P. D. Gianino, �??Applying the phase-only filter concept to the synthetic discriminant function correlation filter,�?? Appl. Opt. 24, 851�??855 (1985). [CrossRef] [PubMed]
- D. P. Casasent and W. A. Rozzi, �??Computer-generated and phase-only synthetic discriminant function filters,�?? Appl. Opt. 25, 3767�??3772 (1986). [CrossRef] [PubMed]
- R. R Kallman, �??Optimal low noise phase-only and binary phase-only optical correlation filters for threshold detectors,�?? Appl. Opt. 25, 4216�??4217 (1986). [CrossRef] [PubMed]
- E. Carcole, M. S. Millan, and J. Campos, �??Derivation of weighting coefficients for multiplexed phase-diffractive elements,�?? Opt. Lett. 20, 2360�??2362 (1995). [CrossRef] [PubMed]

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