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Journal of the Optical Society of America A

Journal of the Optical Society of America A

| OPTICS, IMAGE SCIENCE, AND VISION

  • Editor: Franco Gori
  • Vol. 31, Iss. 8 — Aug. 1, 2014
  • pp: 1842–1846
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Large aperture adaptive doublet polymer lens for imaging applications

Freddie Santiago, Brett Bagwell, Ty Martinez, Sergio Restaino, and Sanjay Krishna  »View Author Affiliations


JOSA A, Vol. 31, Issue 8, pp. 1842-1846 (2014)
http://dx.doi.org/10.1364/JOSAA.31.001842


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Abstract

We report a full design process—finite element modeling, fabrication, and characterization—of adaptive doublet polymer lenses. A first-order model was developed and used to design fluidic doublets, analogous to their glass counterparts. Two constant-volume fluidic chambers were enclosed by three flexible membranes, resulting in a variable focal length doublet with a clear aperture of 19.0 mm. Chromatic focal shift was then used to compare numerical modeling to experimentally measured results over a positive focal length range of 55–200 mm (f/2.89 to f/10.5).

© 2014 Optical Society of America

1. INTRODUCTION

Variable focal length or adaptive lenses have been proposed since the 1800s, with the first patent awarded in 1866 [1

1. D. A. Woodward, “Improvement in fluid lenses,” Letters Patent60,109 (November27, 1866).

]. Over the past 10 years, there has been a resurgence of interest in this type of lens, initially with clear apertures of 5 mm or less, and based on actuation techniques, such as electrowetting, variable fluid volume, and mechanical deformation of a flexible surface [2

2. B. Berge and J. Peseux, “Variable focal lens controlled by an external voltage: an application of electro-wetting,” Eur. Phys. J. E 3, 159–163 (2000). [CrossRef]

4

4. H. Ren and S. Wu, Introduction to Adaptive Lens (Wiley, 2012).

]. Commercial entities such as Holochip and Optotune have subsequently developed larger adaptive lenses, on the order of 10 mm (actuated) and as large as 20 mm (manual) in diameter.

In this context, there have been efforts to move beyond mere singlet lenses, containing only one or two flexible surfaces and single fluid. In one demonstrated configuration, using the electrowetting technique, the immiscible property of two incompatible fluids allowed for two separate fluid volumes, but was inherently aperture limited. A second proposed configuration enclosed two fluids between rigid optical windows, with an intermediate flexible membrane as the only variable, resulting in a low dynamic range due to the limited difference in refractive index across the variable surface. The third effort that we are aware of used two flexible membranes and multiple chambers, but required variable fluid volume control in each chamber [9

9. P. Waibel, D. Mader, P. Liebetraut, H. Zappe, and A. Seifert, “Chromatic aberration control for tunable all-silicone membrane microlenses,” Opt. Express 19, 18584–18592 (2011). [CrossRef]

]. While the latter gives a large dynamic range, it creates practical problems (repeatability and reliability) from an implementation perspective [10

10. A. Miks and J. Novak, “Analysis of two-element zoom systems based on variable power lenses,” Opt. Express 18, 6797–6810 (2010). [CrossRef]

12

12. S. Reichelt and H. Zappe, “Design of spherically corrected, achromatic variable-focus liquid lenses,” Opt. Express 15, 14146–14154 (2007). [CrossRef]

]. All of these configurations present challenges for military applications, namely small aperture sizes, low dynamic range, and impractical actuation modalities.

We present a large aperture (19 mm), constant-volume, variable focal length doublet, where two optical fluids are enclosed and separated by three flexible polymer membranes. By applying pressure to the two outer membranes, we deform all three membranes, changing the radius of curvature (ROC). For this type of configuration, the behavior of the lens is determined not only by the fluid properties, but also by the membrane’s mechanical properties, specifically the relative in-plane tension of each surface generated during the fabrication process.

We developed a first-order model that allowed us to go from an optical design to fabricating the adaptive doublet polymer lenses (ADPLs). This model required understanding the relationship between the pressure inside the lens, the volume enclosed in the lens hemisphere, and the in-plane tension of the membrane. Results from two doublets fabricated via this process are presented.

2. ADAPTIVE POLYMER LENS FABRICATION AND CONFIGURATION

SNL and NRL have developed a process that encompasses finite element modeling, polymer membrane research, development, and fabrication techniques in order to produce the best optical quality adaptive lens possible. Gravity-induced coma has been minimized by controlling the mechanical properties of the membrane, and thermally induced ROC and index changes are controlled by active athermalization [13

13. M. S. Baker, B. J. Anderson, G. Soehnel, B. Bagwell, and F. Santiago, “Polymer adaptive lens athermalization,” Sandia Report SAND10 (Sandia National Laboratories, 2011).

].

Figure 1 shows an ADPL. The lens is composed of rings that act as both the support structure and clear aperture boundary condition for the polydimethylsiloxane (PDMS) membranes. Because of our process, these rings can be made of almost any material. Glass, aluminum, titanium, and silicon carbide have been used.

Fig. 1. (Left) ADPL focusing on a business card. (Right) Side view of the ADPL showing support rings.

Once the lens is made, it is inserted in the piezoelectric actuation mechanism that axially compresses the membrane, resulting in deformation. The actuator has a positional resolution of 40nm, and no power is consumed once the desired ROC is achieved. Figure 8 shows a cross-sectional representation of an actuated ADPL.

In order to produce an ADPL with achromatic characteristics, fluids with crown and flint-like properties were selected for their index (n) and Abbe number (V). For the crown fluid n1=1.44 and V1=62.2, and for the flint n2=1.58 and V2=29.0. Zemax was used to design an ADPL using these fluids. Figure 2 shows an example of the ray-tracing sketch of the doublet, as well as the chromatic focal shift (at one focal length).

Fig. 2. Chromatic focal shift plot for a designed ADPL and ray-tracing diagram.

3. FROM DESIGN TO FABRICATION

In order to move from the optical design to fabrication, an understanding of the relationship between the pressure inside the lens, the volume of the encapsulated hemisphere, and the resultant ROCs needs to be established.

A. Model: Linear Approximation

A finite element model (FEM) was developed to obtain the relationship between the pressure and volume of the spherical cap (Vcap) as a function of the membrane thickness. Using this relationship, the ROC can be determined analytically from the volume within the spherical cap. This relationship allows us to relate fabrication variables (thickness) to optical design parameters (ROC). The relationship between ROC and Vcap can be seen in Fig. 3, and is inversely proportional to ROC:
Vcap=γ·1ROC,
(1)
where γ is a proportionality factor.

Fig. 3. Conceptual drawing showing the relationship between ROC and Vcap.

We define the “thickness ratio” as
Tr=TiTf,
(2)
where Ti is the initial thickness and Tf is the final thickness of the membrane once the in-plane stress has been applied. A uniform thin membrane was modeled in the FEM. Boundary conditions were established at the appropriate radii, and a pressure differential was created across the membrane for a given Tr. Figure 4 shows results from the FEM as well as a linear fit for three distinct Tr.

Fig. 4. Pressure versus volume results obtained from FEM and their respective linear fits.

To determine the validity of these results, an experiment was designed to mimic the conditions of the FEM. The fixture shown in Fig. 5 was fabricated to encapsulate a membrane with a given Tr. A controlled pressure differential was applied to the membrane, and Vcap and ROC were measured in situ with a Zygo interferometer. Figure 6 shows the experimental results obtained for Tr, similar to that used in the FEM, as well as the fits obtained.

Fig. 5. Fixture used to measure pressure versus volume and ROC, as a function of Tr. The black port connects to a pressure gauge and measures the pressure inside the lens, and the clear port controls the volume and allows us to change the ROC.
Fig. 6. Pressure versus volume results obtained from experimental results and their respective linear fits.

Figure 7 shows a plot of the FEM and the experimental data for Tr=0.76. The high degree of concurrence between the model and measured data validated our first-order approach.

Fig. 7. Comparison between the FEM and experimental results for a thickness ratio of Tr=0.77.
Fig. 8. Cross-sectional representation of an ADPL and pressure, volume, and thickness ratio relationship.

The experimental results confirmed a linear relationship between the change in pressure and the spherical cap:
ΔP=m(Tr)*Vcap,
(3)
where m is a function of Tr. The relationship between m and Tr was obtained empirically from the slopes of each of the Tr and is of the form
m(Tr)=α*Tr+β,
(4)
where α and β are empirically calculated constants and are dependent on the membrane’s mechanical properties. For this case, the material is PDMS and they are equal to 499.48 and 484.22, respectively.

By knowing Tr and the desired ROC, m can be calculated and the relationship between pressure and volume for a membrane of a particular thickness can be determined. In order to model a multiple flexible membrane lens, we need to apply a constant-volume condition, and express the differential in pressure across the membranes by
ΔPT=n=1MΔPn=n=1Mmn*Vcapn=0,
(5)
where M is the total number of membranes and ΔPn represents the pressure differential at each boundary. Figure 8 shows a cross-sectional schematic of a three-membrane lens.

From the ADPL prescription we obtain three ROCs and their associated Vcap. We then pick two thickness (ratios), which dictates their m(Tr). Using Eq. (5) we solve for the remaining thickness ratio slope, m(Tr). From m(Tr), the remaining Tr can be determined from Eq. (4). Figure 9 illustrates a flow diagram of the design process.

Fig. 9. Optical design to fabrication process for an ADPL.

4. OPTICAL SETUP AND EXPERIMENTAL RESULTS

A. ROC and Back Focal Length Determination

Fig. 10. Graphical representation of the measured prescription of ADPLs 1 and 2.

An optical benchtop setup was designed in which three lasers, of wavelengths 632, 532, and 405 nm, were expanded to fully illuminate the 19 mm clear aperture of the ADPL. A charge-coupled device (CCD) was placed on an optical translation stage with a micrometer, and one beam at a time was used to find the best focus. Once the best focus was identified, the position was recorded. The process was then repeated for the other two wavelengths, and the back focal length was measured as well. The APDL was then driven to another ROC, and the process was repeated. Figure 11 shows the optical setup.

Fig. 11. Optical setup with three collimated beams at the wavelengths of interest, 632, 532, and 405 nm.

B. Chromatic Focal Shift Results

Table 1 shows the experimental and theoretical results for the two fabricated ADPLs.

Table 1. Actuated ADPL Focal Shift Results for Two Achromatic Doublets Obtained from Measurements and Zemax Calculations with Their Respective Effective Focal Lengths (EFFLs)a

table-icon
View This Table

The average focal shift measured on the ADPL was 4.13 mm for B-R and 0.87 mm for the G-R band. For both doublets, the calculated and measured G-R shifts are in better agreement than the B-R shifts.

5. CONCLUSIONS

We have demonstrated a first-order model that allows a three-surface, two-chamber, constant-volume, variable focal length lens to be fabricated. Via design optimization and characterization, we demonstrated that achromatization was achievable through the full dynamic range of the lens.

ACKNOWLEDGMENTS

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND Number: 2014-3846 J.

REFERENCES

1.

D. A. Woodward, “Improvement in fluid lenses,” Letters Patent60,109 (November27, 1866).

2.

B. Berge and J. Peseux, “Variable focal lens controlled by an external voltage: an application of electro-wetting,” Eur. Phys. J. E 3, 159–163 (2000). [CrossRef]

3.

S. Kuiper and B. H. W. Hendriks, “Variable-focus liquid lens for miniature cameras,” Appl. Phys. Lett. 85, 1128–1130 (2004). [CrossRef]

4.

H. Ren and S. Wu, Introduction to Adaptive Lens (Wiley, 2012).

5.

G. Beadie, M. L. Sandrock, M. J. Wiggins, R. S. Lepkowicz, J. S. Shirk, M. Ponting, Y. Yang, T. Kazmierczak, A. Hiltner, and E. Baer, “Tunable polymer lens,” Opt. Express 16, 11847–11857 (2008). [CrossRef]

6.

B. E. Bagwell and F. Santiago, “RD100: RAZAR adaptive zoom rifle scope,” RD100 (2014).

7.

D. V. Wick, “Active optical zoom system,” U.S. patent6,977,777 (December20, 2005).

8.

D. V. Wick, T. Martinez, D. M. Payne, W. C. Sweatt, and S. R. Restaino, “Active optical zoom system,” Proc. SPIE 5798, 151–157 (2005). [CrossRef]

9.

P. Waibel, D. Mader, P. Liebetraut, H. Zappe, and A. Seifert, “Chromatic aberration control for tunable all-silicone membrane microlenses,” Opt. Express 19, 18584–18592 (2011). [CrossRef]

10.

A. Miks and J. Novak, “Analysis of two-element zoom systems based on variable power lenses,” Opt. Express 18, 6797–6810 (2010). [CrossRef]

11.

A. Miks and J. Novak, “Analysis of three-element zoom lens based on refractive variable-focus lenses,” Opt. Express 19, 23989–23996 (2011). [CrossRef]

12.

S. Reichelt and H. Zappe, “Design of spherically corrected, achromatic variable-focus liquid lenses,” Opt. Express 15, 14146–14154 (2007). [CrossRef]

13.

M. S. Baker, B. J. Anderson, G. Soehnel, B. Bagwell, and F. Santiago, “Polymer adaptive lens athermalization,” Sandia Report SAND10 (Sandia National Laboratories, 2011).

OCIS Codes
(220.0220) Optical design and fabrication : Optical design and fabrication
(220.3630) Optical design and fabrication : Lenses
(220.4610) Optical design and fabrication : Optical fabrication
(250.0250) Optoelectronics : Optoelectronics
(250.2080) Optoelectronics : Polymer active devices
(220.1080) Optical design and fabrication : Active or adaptive optics

ToC Category:
Optical Design and Fabrication

History
Original Manuscript: May 16, 2014
Manuscript Accepted: June 26, 2014
Published: July 28, 2014

Virtual Issues
July 31, 2014 Spotlight on Optics

Citation
Freddie Santiago, Brett Bagwell, Ty Martinez, Sergio Restaino, and Sanjay Krishna, "Large aperture adaptive doublet polymer lens for imaging applications," J. Opt. Soc. Am. A 31, 1842-1846 (2014)
http://www.opticsinfobase.org/josaa/abstract.cfm?URI=josaa-31-8-1842


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References

  1. D. A. Woodward, “Improvement in fluid lenses,” Letters Patent60,109 (November27, 1866).
  2. B. Berge and J. Peseux, “Variable focal lens controlled by an external voltage: an application of electro-wetting,” Eur. Phys. J. E 3, 159–163 (2000). [CrossRef]
  3. S. Kuiper and B. H. W. Hendriks, “Variable-focus liquid lens for miniature cameras,” Appl. Phys. Lett. 85, 1128–1130 (2004). [CrossRef]
  4. H. Ren and S. Wu, Introduction to Adaptive Lens (Wiley, 2012).
  5. G. Beadie, M. L. Sandrock, M. J. Wiggins, R. S. Lepkowicz, J. S. Shirk, M. Ponting, Y. Yang, T. Kazmierczak, A. Hiltner, and E. Baer, “Tunable polymer lens,” Opt. Express 16, 11847–11857 (2008). [CrossRef]
  6. B. E. Bagwell and F. Santiago, “RD100: RAZAR adaptive zoom rifle scope,” (2014).
  7. D. V. Wick, “Active optical zoom system,” U.S. patent6,977,777 (December20, 2005).
  8. D. V. Wick, T. Martinez, D. M. Payne, W. C. Sweatt, and S. R. Restaino, “Active optical zoom system,” Proc. SPIE 5798, 151–157 (2005). [CrossRef]
  9. P. Waibel, D. Mader, P. Liebetraut, H. Zappe, and A. Seifert, “Chromatic aberration control for tunable all-silicone membrane microlenses,” Opt. Express 19, 18584–18592 (2011). [CrossRef]
  10. A. Miks and J. Novak, “Analysis of two-element zoom systems based on variable power lenses,” Opt. Express 18, 6797–6810 (2010). [CrossRef]
  11. A. Miks and J. Novak, “Analysis of three-element zoom lens based on refractive variable-focus lenses,” Opt. Express 19, 23989–23996 (2011). [CrossRef]
  12. S. Reichelt and H. Zappe, “Design of spherically corrected, achromatic variable-focus liquid lenses,” Opt. Express 15, 14146–14154 (2007). [CrossRef]
  13. M. S. Baker, B. J. Anderson, G. Soehnel, B. Bagwell, and F. Santiago, “Polymer adaptive lens athermalization,” (Sandia National Laboratories, 2011).

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