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Optics Letters

Optics Letters


  • Editor: Alan E. Willner
  • Vol. 35, Iss. 16 — Aug. 15, 2010
  • pp: 2735–2737
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Endoscope lens with dual fields of view and resolutions for multiphoton imaging

Minghan Chen, Chris Xu, and Watt W. Webb  »View Author Affiliations

Optics Letters, Vol. 35, Issue 16, pp. 2735-2737 (2010)

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We report the development of a miniaturized dual-optical-zone endoscope objective lens. The lens has two foci, with 0.18 and 0.50 NAs. We demonstrate multiphoton imaging with dual fields of view and resolutions using the new lens. A combination of multiphoton and single-photon microscopic imaging is also demonstrated.

© 2010 Optical Society of America

The lens assembly was designed with Zemax software, and the ray-tracing diagram is illustrated in Fig. 1a. Light emitting from a point source is focused by the lens assembly into two foci simultaneously with two different NAs. The lens assembly includes a collimating lens and a bifocal lens. The collimating lens was fabricated from a commercial lens (Edmund Optics NT46-342) by cutting down the peripheral areas of the lens so that a diameter of 4mm is achieved. The bifocal lens was fabricated by first cutting an asphere lens (Edmund Optics NT-46343) into 4mm diameter. Then, a center through-hole (5mm length) was drilled, and a GRIN objective (GRINTECH GmbH) of 5mm length was cemented into the through hole. The collimation lens and the focusing lens are in physical contact with each other. Figure 1b shows the fabricated bifocal objective lens assembly. The lens outer diameter is 4.0mm. The central optical zone is around 2mm diameter, while the peripheral optical zone covers the remaining lens area. For a focal point with the 0.5 NA central zone, the Strehl ratios are 0.98, 0.50, and 0.25 at the center, 0.7, and full FOV (188μm), respectively, which are typical of a GRIN objective. For a focal point with the 0.18 NA peripheral zone, the Strehl ratios are 0.89, 0.71 and 0.3 at the center, 0.7, and full FOV, respectively. At the peripheral area of the FOV, astigmatism and coma are the major aberrations. We note that the bifocal lens assembly was fabricated using low-cost, commercially available lenses to demonstrate the feasibility of optical zooming. Off-axis aberrations can be significantly reduced by using customized lenses.

Using 800nm center wavelengths and assuming a 10nm bandwidth, the calculated focal plane axial chromatic shifts are 1.4 and 8μm for the 0.5 and 0.18 NA foci, respectively. Maximal lateral colors (lateral chromatic shift of the focus) at the image plane (at the largest FOV) are 0.006 and 0.02μm for the 0.18 and 0.5 NA foci, respectively. These values are much smaller than the sizes of both foci. Furthermore, the values are significantly smaller than the pulse length in space (e.g., 30μm for a 100fs pulse). Thus, chromatic aberration will not cause significant spatial and temporal distortion at the focus.

Figure 2 shows the instrument setup for acquiring reflection/multiphoton images with large FOV/low resolution and small FOV/high resolution. Switching between the two imaging modes does not require any optical or mechanical alteration of the imaging system, enabling a compact and convenient configuration for MPM-E. By adjusting the relative distance between the endoscope assembly and the sample along the optical axis without lateral motion, the high-resolution image obtained was exactly at the center of the larger FOV image. In practice, lateral motion may need to be incorporated in the translation process to account for the possible motion of the object.

To characterize the lateral resolution, we examined the endoscope objective lens with a U.S. Air Force (USAF) target using one-photon reflection imaging. Figure 3a shows the measured reflection image of a USAF target. To characterize the resolution limit of the lens, magnified images were obtained by scanning a small area in the center of the FOV. With a 0.18 NA, the simulated resolution is 2.14μm, which is close to the width of the group 7 element 6 (2.19μm) [highlighted using the dashed rectangle in Fig. 3a]. Reflection images with the higher resolution (0.50 NA) were also measured with bars in groups 8 and 9 [Fig. 3b]. As indicated with the dashed rectangle in Fig. 3b, the bar width of 0.977μm can be resolved, which agrees very well with the theoretical resolution of 0.976μm for a 0.5 NA optical system using 800nm laser light.

Because the two foci of the bifocal lens are located coaxially along the optical axis, cross talk between the foci is a concern. The inherent axial sectioning capability of MPM [1

1. W. Denk, J. H. Strickler, and W. W. Webb, Science 248, 73 (1990). [CrossRef] [PubMed]

] ensures that there is negligible out-of-focus background generation, removing the cross talk between the two imaging modes. Thus, multiphoton imaging is naturally well suited for such a bifocal lens design. We characterized the lens axial resolution of the optical system by scanning through the center of 0.2μm beads along the axial direction using a translation stage. As shown in Fig. 4, two foci were observed along the optical axis, with a separation of 7.8mm. The fluorescence intensity of each focus was normalized and fitted with a Gaussian profile shown by the insets. The FWHM of the axial response for the 0.5 and 0.18 NA optical zones are 5.8 and 56μm, respectively. The FWHM value of the 0.5 NA focus is close to the theoretical prediction of 4.63μm. The small difference between the experiment and the theory is likely due to the chromatic aberration of the lens assembly. Figure 4 clearly shows that the cross talk is negligible, and the presence of the second focus has negligible impact on multiphoton imaging. Similar to multiphoton imaging, a confocal pinhole can be added to reduce the cross talk of the two foci in single-photon imaging.

In summary, a dual optical zone lens was designed, fabricated, and characterized. The feasibility of employing the dual-zone endoscope lens to achieve multiphoton/ single-photon imaging with dual FOVs and resolutions was demonstrated. Such a dual-zone lens system could provide a practical solution for clinical in vivo imaging by locating diseased areas with its larger FOV and subsequently providing high-resolution clinical diagnosis without the need for any optical or mechanical adjustment of the endoscope.

This research was supported by the National Institutes of Health (NIH) grant 1 R01 EB006736-03. The authors would like to thank Mark Williams for the critical reading and insightful comments.

Fig. 1 (a) Ray tracing diagram of the optical properties of the bifocal lens assembly. (b) Top view of a bifocal lens assembly in a lens holder. The center and peripheral optical zones are indicated by the image circular truncation of an underlying grid pattern.
Fig. 2 Schematic diagram of the experiment setup for one-photon reflectance and multiphoton imaging.
Fig. 3 Reflection images of a USAF target measured with NAs of (a) 0.18 and (b) 0.5. The insets in (a) and (b) are magnified images of the bars highlighted by the square. For all images, a 1Hz frame rate was used.
Fig. 4 Fluorescence intensity axial profile (in logarithmic scale) of both foci through the center of a fluorescence bead along the axial direction. Normalized peak intensities with a solid line Gaussian fit are shown in inset figures.
Fig. 5 Multiphoton images of 6μm beads fixed in agarose measured with 0.18 and 0.50 NA in (a) and (b), respectively. The horizontal scale bars are 100μm.
Fig. 6 (a),(b) SHG and (c),(d) reflection images of a rat-tail tendon, acquired with (b),(d) 0.50 and (a),(c) 0.18 NA. The scale bars are 100μm.

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W. R. Zipfel, R. M. Williams, and W. W. Webb, Nature Biotechnol. 21, 1369 (2003). [CrossRef]


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L. Fu, X. Gan, and M. Gu, Opt. Express 135528 (2005). [CrossRef] [PubMed]


R. Le Harzic, M. Weinigel, I. Riemann, K. König, and B. Messerschmidt, Opt. Express 16, 20588 (2008). [CrossRef] [PubMed]


Y. Wu, J. Xi, M. J. Cobb, and X. D. Li, Opt. Lett. 34, 953 (2009). [CrossRef] [PubMed]


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OCIS Codes
(120.3890) Instrumentation, measurement, and metrology : Medical optics instrumentation
(170.3880) Medical optics and biotechnology : Medical and biological imaging
(190.4180) Nonlinear optics : Multiphoton processes
(180.4315) Microscopy : Nonlinear microscopy

ToC Category:
Medical Optics and Biotechnology

Original Manuscript: March 22, 2010
Revised Manuscript: June 11, 2010
Manuscript Accepted: July 13, 2010
Published: August 12, 2010

Virtual Issues
Vol. 5, Iss. 13 Virtual Journal for Biomedical Optics

Minghan Chen, Chris Xu, and Watt W. Webb, "Endoscope lens with dual fields of view and resolutions for multiphoton imaging," Opt. Lett. 35, 2735-2737 (2010)

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