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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 14 — Jul. 5, 2010
  • pp: 14745–14751
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Multiple-channel spectrally encoded imaging

Avraham Abramov, Limor Minai, and Dvir Yelin  »View Author Affiliations


Optics Express, Vol. 18, Issue 14, pp. 14745-14751 (2010)
http://dx.doi.org/10.1364/OE.18.014745


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Abstract

Spectrally encoded endoscopy (SEE) uses miniature diffractive optics to encode space with wavelength, allowing video-rate three-dimensional imaging through sub-millimeter, flexible endoscopic probes. Here we present a new approach for SEE in which the illumination and the collection channels are separated in space, and spectral encoding is present only in the collection channel. Bench-top experiments using spatially incoherent white light illumination reveal significant improvement in image quality and considerable reduction of speckle noise compared to conventional techniques, and show that the new system is capable of high sensitivity fluorescence imaging of single cells. The presented new approach would allow improved functionality and usability of SEE.

© 2010 OSA

1. Introduction

Miniaturization of instrumentation for minimally invasive clinical intervention is a current trend in medicine, pressed forward by the constant advance in science and technology. The recent progress in solid state imaging technology allows current state-of-the-art endoscopes to perform various intervention and surgical procedures guided by high-resolution, real-time imaging deep inside the body. Since the mid-80’s, the remarkable progress in fiber optics and photonics technologies have opened new opportunities for imaging inside the body through more compact endoscopic instruments, owing to the small diameter and flexibility of silica optical fibers. Fiber bundle endoscopes [1

1. B. I. Hirschowitz, “Endoscopic examination of the stomach and duodenal cap with the fiberscope,” Lancet Infect. Dis. 1, 1074–1078 (1961).

4

4. M. A. D’Hallewin, S. El Khatib, A. Leroux, L. Bezdetnaya, and F. Guillemin, “Endoscopic confocal fluorescence microscopy of normal and tumor bearing rat bladder,” J. Urol. 174(2), 736–740 (2005). [CrossRef] [PubMed]

] have made a considerable impact in clinical applications such as ductoscopy [5

5. V. R. Jacobs, M. Kiechle, B. Plattner, T. Fischer, and S. Paepke, “Breast ductoscopy with a 0.55-mm mini-endoscope for direct visualization of intraductal lesions,” J. Minim. Invasive Gynecol. 12(4), 359–364 (2005). [CrossRef] [PubMed]

,6

6. E. R. Sauter, H. Ehya, L. Schlatter, and B. MacGibbon, “Ductoscopic cytology to detect breast cancer,” Cancer J. 10(1), 33–41, discussion 15–16 (2004). [CrossRef] [PubMed]

], embryoscopy [7

7. E. A. Reece, “Embryoscopy and early prenatal diagnosis,” Obstet. Gynecol. Clin. North Am.24, 111–121 (1997). [CrossRef] [PubMed]

], and angioscopy [8

8. A. J. Saltzman and S. Waxman, “Angioscopy and ischemic heart disease,” Curr. Opin. Cardiol. 17(6), 633–637 (2002). [CrossRef] [PubMed]

] which require imaging through small diameter probes. The use of a single optical fiber and a scanning mechanism at the distal end of an endoscope [9

9. C. M. Brown, P. G. Reinhall, S. Karasawa, and E. J. Seibel, “Optomechanical design and fabrication of resonant microscanners for a scanning fiber endoscope,” Opt. Eng. 45(4), 043001 (2006). [CrossRef]

11

11. Y. C. Wu, Y. X. Leng, J. F. Xi, and X. D. Li, “Scanning all-fiber-optic endomicroscopy system for 3D nonlinear optical imaging of biological tissues,” Opt. Express 17(10), 7907–7915 (2009). [CrossRef] [PubMed]

] eliminate pixelation artifacts and improve probe flexibility, at the expense, however, of the bulk of the mechanical scanning apparatus at the distal end of the endoscope. Spectrally encoded endoscopy (SEE), first presented in 1998 by Tearney et al. [12

12. G. J. Tearney, R. H. Webb, and B. E. Bouma, “Spectrally encoded confocal microscopy,” Opt. Lett. 23(15), 1152–1154 (1998). [CrossRef]

], uses wavelengths from a broadband source to encode a single lateral axis on the sample, while the second transverse dimension is scanned by slow probe rotation. SEE has been shown promising for high speed confocal microscopy [13

13. D. Yelin, C. Boudoux, B. E. Bouma, and G. J. Tearney, “Large area confocal microscopy,” Opt. Lett. 32(9), 1102–1104 (2007). [CrossRef] [PubMed]

15

15. K. Goda, K. K. Tsia, and B. Jalali, “Serial time-encoded amplified imaging for real-time observation of fast dynamic phenomena,” Nature 458(7242), 1145–1149 (2009). [CrossRef] [PubMed]

] and for video-rate three dimensional endoscopic imaging through sub-millimeter, flexible probes [16

16. L. Froehly, S. N. Martin, T. Lasser, C. Depeursinge, and F. Lang, “Multiplexed 3D imaging using wavelength encoded spectral interferometry: a proof of principle,” Opt. Commun. 222(1-6), 127–136 (2003). [CrossRef]

18

18. D. Yelin, W. M. White, J. T. Motz, S. H. Yun, B. E. Bouma, and G. J. Tearney, “Spectral-domain spectrally-encoded endoscopy,” Opt. Express 15(5), 2432–2444 (2007). [CrossRef] [PubMed]

].

In its current mode of implementation [17

17. D. Yelin, I. Rizvi, W. M. White, J. T. Motz, T. Hasan, B. E. Bouma, and G. J. Tearney, “Three-dimensional miniature endoscopy,” Nature 443(7113), 765 (2006). [CrossRef] [PubMed]

], SEE has several limiting factors which need to be addressed before its clinical promise could be realized. First, the use of wavelength to encode space imposes some difficulties on wavelength-sensitive imaging modalities. For example, fluorescence spectrally encoded imaging required a sophisticated optical setup for frequency-encoding [19

19. J. T. Motz, D. Yelin, B. J. Vakoc, B. E. Bouma, and G. J. Tearney, “Spectral- and frequency-encoded fluorescence imaging,” Opt. Lett. 30(20), 2760–2762 (2005). [CrossRef] [PubMed]

]. Additionally, the use of spatially coherent illumination through a single mode fiber causes pronounced speckle noise, small depth of field, and poor signal collection efficiency which often requires the use of lasers, supercontinuum generation sources, or high power super-luminescent diode arrays. One possible solution for addressing these issues includes the use of a double-clad fiber [20

20. D. Yelin, B. E. Bouma, S. H. Yun, and G. J. Tearney, “Double-clad fiber for endoscopy,” Opt. Lett. 29(20), 2408–2410 (2004). [CrossRef] [PubMed]

] for spatially coherent sample illumination and incoherent signal collection. While double-clad SEE was demonstrated capable of speckle-free imaging with large depth of field, the endoscopic probe itself suffered from significant cross-talk between the illumination and the collection channels. Back reflections from the probe’s optics, which were efficiently collected by the large area and the high numerical aperture of the inner cladding, resulted with high image noise and required continuous background subtraction during image acquisition.

2. Multiple-channel SEE

Here we show a new approach for SEE, termed multiple-channel SEE (MC-SEE), which addresses many of the image quality concerns of SEE and further expands its functionality. While current forms of SEE involve spectral encoding in both the collection and illumination channels independently, effective encoded imaging is still feasible using only one encoded channel. The concept of imaging with a single encoded channel is schematically illustrated in Fig. 1
Fig. 1 Single channel spectral encoding. Schematic drawing of the optical paths in single channel space-to-wavelength encoding, illustrating the collection of a single wavelength from each lateral point on the sample.
, showing space-to-wavelength encoding of broadband light (e.g. fluorescence) emanating from a specimen.

Several bench-top experiments were conducted in order to study different sample illumination configurations (Fig. 2
Fig. 2 Schematic drawing of the experimental setups with front, back, and spectrally encoded illumination.
): wide-field (not encoded) front illumination, wide-field back illumination simulating diffuse light emanating from the tissue surface, and spectrally encoded illumination which utilized the imaging lens-grating pair but delivered through a separate multi-mode optical fiber. The spectrally encoded collection channel was comprised of a single-mode optical fiber (Nufern, S405-HP), an imaging lens (25 mm focal length, 25 mm diameter), a transmission diffraction grating (Wasatch photonics, 1200 lines/mm, maximum diffraction efficiency at 570 nm), and a spectrometer comprised of a collimation objective lens (Leica, Achro 0.1 NA), a diffraction grating (Wasatch photonics, 1800 lines/mm, maximum diffraction efficiency at 532 nm), a multi-element lens (Nikon AFC, 50 mm focal length) and a high sensitivity back-illuminated electron multiplication charged coupled device (EMCCD) camera (Andor, DU970N-BV). Both grating-lens pairs at the two ends of the fiber were optimized to transmit wavelengths in the range of 450-650 nm, with 550 nm at Littrow’s angle.

Front and back illumination of the sample were accomplished by directing a beam of white light (150W Halogen lamp) to the sample using a fiber bundle light guide (Schott ACE, A20500). The total power illuminating circular spot approximately 5 mm in diameter on the sample was 150 mW in the visible and the near infrared regions. We estimate that less than 1% of the illumination was overlapping with the imaged spectrally encoded line. Scanning in the direction perpendicular to the spectrally encoded line (x-axis) was achieved by moving the sample using a motorized translation stage. Exposure time was 100 ms per single line. MC-SEE images of a USAF-1951 resolution target printed in black ink on white paper (Newport, RES-2) using front and back sample illumination are shown in Figs. 3a
Fig. 3 Demonstration of different MC-SEE illumination configurations. a. An image of a paper resolution target using front illumination with visible light. b. Same as (a), using back illumination. c. A portion of a coin surface using spectrally encoded spatially coherent illumination in the near infrared (a Ti:sapphire laser). d. Same as (c), using wide field incoherent front illumination. Scale bars correspond to 1mm.
and 3b, respectively. The images look nearly identical, with negligible speckle noise and resolution of approximately 64 line-pairs per millimeter (group 6, element 1) at the center field of view, which was limited primarily by the number of pixels (1600 horizontal pixels) in the spectrometer’s EMCCD. Some blurring, ghost pattern and a drop in resolution by a factor of 1.5 is noticeable in Figs. 3a and 3b, most likely caused by optical aberrations in the spectrometer’s grating. In order to demonstrate the advantages of spatially incoherent illumination over coherent illumination, the system was readjusted to employ titanium-sapphire laser illumination (Femtolasers Produktions GmbH, Rainbow) and the experimental setup was optimized for the near-infrared region of the spectrum (650-950 nm), including the replacement of the spectrometer’s grating (Wasatch photonics, 1800 lines/mm, maximum diffraction efficiency at 840 nm), the imaging grating (Wasatch photonics, 1200 lines/mm, maximum diffraction efficiency at 830 nm), and the single-mode optical fiber (Nufern, 780-HP). MC-SEE images of a portion of a 1 Euro cent coin using spectrally encoded coherent and incoherent front illumination are shown in Fig. 3c and Fig. 3d, respectively. Comparison between the two images reveal significant reduction in speckle noise using incoherent illumination (Fig. 3d), resulting in a more natural appearance with better discrimination of surface texture.

3. Fluorescence MC-SEE imaging

4. Discussion

The presented new approach for spectrally encoded imaging addresses several key challenges in this technology, including high back reflections from the probes, inefficient fluorescence encoding, and pronounced speckle noise. Using only a single encoded channel in the collection path, back-reflections from interfaces within the imaging probe could be minimized, as well as undesired coupling of the illumination light into the collection optical path. As a consequence of the low background signal, high sensitivity low noise cameras could be utilized. In our experiments, an electrically cooled EMCCD camera was used, capable of line rates of up to 1500 Hz, potentially allowing real-time, high resolution imaging. When illuminations is efficient and signal levels are sufficiently high, our spectrometer allows capturing speeds of up to 2.5 frames per second, where each frame contains 800 x 600 pixels. Using white light incoherent illumination, MC-SEE images show significant improvement in quality with complete elimination of speckle noise and natural appearance. Similar image characteristics were previously obtained using SEE with a single double-clad fiber [20

20. D. Yelin, B. E. Bouma, S. H. Yun, and G. J. Tearney, “Double-clad fiber for endoscopy,” Opt. Lett. 29(20), 2408–2410 (2004). [CrossRef] [PubMed]

], however, the use of such fibers often required tedious alignment procedures of the optical setup, including the use of cross-polarizers to eliminate reflections from the fiber interfaces, spatial mode filtering to reduce coherent light collection through the fiber’s core, and real-time background subtraction due to varying back-reflections from the probe optics (which could not prevent the increase in shot noise).

However, low signal levels such as those emitted from fluorescence markers, would require longer exposure times and could reduce frame rates. A main cause for the relatively weak fluorescence signals is the fairly inefficient signal collection: using wavelength-encoding for imaging resulted in collection of only a small fraction /σλ = 1/Nx of the broadband fluorescence signal, where denotes the local bandwidth at each resolvable point. Several straightforward measures could be taken in order to improve signal efficiency, including the incorporation of a more intense spatially coherent supercontinuum source, and the use of dedicated line cameras specifically designed for the detection of low light levels at high line rates. When using less power-efficient broadband sources such as those used in this work, additional optics in the illumination channel such as cylindrical lenses, optical diffusers and diffraction gratings would improve spatial overlapping between the illumination and the imaged line. The physical characteristics of the fluorescence marker dictate not only its effective brightness, but also determine the field of view along the wavelength axis ΔX, which is given by:
ΔX=Gf1σλemcosθ0,
(4)
where σλemdenotes the emission spectrum of the fluorophore. Increasing the field of view would be possible by using fluorescent markers with broader emission spectra or, alternatively, by using multiple fluorophores with overlapping excitation spectra and complementary emission spectra, for labeling similar sites at the sample.

In summary, a new approach for spectrally encoded imaging is experimentally demonstrated, utilizing a single spectrally encoded imaging channel and a separate illumination channel. In addition to providing improved image quality, this technique is demonstrated useful for fluorescence imaging using a simple optical setup. Separate illumination and imaging channels would allow new endoscopic imaging capabilities using MC-SEE, including multiphoton fluorescence and high harmonics imaging, efficient scanningless endoscopic imaging [22

22. K. K. Tsia, K. Goda, D. Capewell, and B. Jalali, “Simultaneous mechanical-scan-free confocal microscopy and laser microsurgery,” Opt. Lett. 34(14), 2099–2101 (2009). [CrossRef] [PubMed]

], and other hybrid imaging modalities.

Acknowledgments

The study was funded in part by the Israel Science Foundation grant (716/09) and by the European Research Council starting grant (239986). The authors thank Dr. Daphne Weihs and Naama Gal for preparing the cell culture samples.

References and links

1.

B. I. Hirschowitz, “Endoscopic examination of the stomach and duodenal cap with the fiberscope,” Lancet Infect. Dis. 1, 1074–1078 (1961).

2.

C. Liang, K. B. Sung, R. R. Richards-Kortum, and M. R. Descour, “Design of a high-numerical-aperture miniature microscope objective for an endoscopic fiber confocal reflectance microscope,” Appl. Opt. 41(22), 4603–4610 (2002). [CrossRef] [PubMed]

3.

A. F. Gmitro and D. Aziz, “Confocal microscopy through a fiber-optic imaging bundle,” Opt. Lett. 18(8), 565–567 (1993). [CrossRef] [PubMed]

4.

M. A. D’Hallewin, S. El Khatib, A. Leroux, L. Bezdetnaya, and F. Guillemin, “Endoscopic confocal fluorescence microscopy of normal and tumor bearing rat bladder,” J. Urol. 174(2), 736–740 (2005). [CrossRef] [PubMed]

5.

V. R. Jacobs, M. Kiechle, B. Plattner, T. Fischer, and S. Paepke, “Breast ductoscopy with a 0.55-mm mini-endoscope for direct visualization of intraductal lesions,” J. Minim. Invasive Gynecol. 12(4), 359–364 (2005). [CrossRef] [PubMed]

6.

E. R. Sauter, H. Ehya, L. Schlatter, and B. MacGibbon, “Ductoscopic cytology to detect breast cancer,” Cancer J. 10(1), 33–41, discussion 15–16 (2004). [CrossRef] [PubMed]

7.

E. A. Reece, “Embryoscopy and early prenatal diagnosis,” Obstet. Gynecol. Clin. North Am.24, 111–121 (1997). [CrossRef] [PubMed]

8.

A. J. Saltzman and S. Waxman, “Angioscopy and ischemic heart disease,” Curr. Opin. Cardiol. 17(6), 633–637 (2002). [CrossRef] [PubMed]

9.

C. M. Brown, P. G. Reinhall, S. Karasawa, and E. J. Seibel, “Optomechanical design and fabrication of resonant microscanners for a scanning fiber endoscope,” Opt. Eng. 45(4), 043001 (2006). [CrossRef]

10.

D. L. Dickensheets and G. S. Kino, “Micromachined scanning confocal optical microscope,” Opt. Lett. 21(10), 764–766 (1996). [CrossRef] [PubMed]

11.

Y. C. Wu, Y. X. Leng, J. F. Xi, and X. D. Li, “Scanning all-fiber-optic endomicroscopy system for 3D nonlinear optical imaging of biological tissues,” Opt. Express 17(10), 7907–7915 (2009). [CrossRef] [PubMed]

12.

G. J. Tearney, R. H. Webb, and B. E. Bouma, “Spectrally encoded confocal microscopy,” Opt. Lett. 23(15), 1152–1154 (1998). [CrossRef]

13.

D. Yelin, C. Boudoux, B. E. Bouma, and G. J. Tearney, “Large area confocal microscopy,” Opt. Lett. 32(9), 1102–1104 (2007). [CrossRef] [PubMed]

14.

C. Boudoux, S. H. Yun, W. Y. Oh, W. M. White, N. V. Iftimia, M. Shishkov, B. E. Bouma, and G. J. Tearney, “Rapid wavelength-swept spectrally encoded confocal microscopy,” Opt. Express 13(20), 8214–8221 (2005). [CrossRef] [PubMed]

15.

K. Goda, K. K. Tsia, and B. Jalali, “Serial time-encoded amplified imaging for real-time observation of fast dynamic phenomena,” Nature 458(7242), 1145–1149 (2009). [CrossRef] [PubMed]

16.

L. Froehly, S. N. Martin, T. Lasser, C. Depeursinge, and F. Lang, “Multiplexed 3D imaging using wavelength encoded spectral interferometry: a proof of principle,” Opt. Commun. 222(1-6), 127–136 (2003). [CrossRef]

17.

D. Yelin, I. Rizvi, W. M. White, J. T. Motz, T. Hasan, B. E. Bouma, and G. J. Tearney, “Three-dimensional miniature endoscopy,” Nature 443(7113), 765 (2006). [CrossRef] [PubMed]

18.

D. Yelin, W. M. White, J. T. Motz, S. H. Yun, B. E. Bouma, and G. J. Tearney, “Spectral-domain spectrally-encoded endoscopy,” Opt. Express 15(5), 2432–2444 (2007). [CrossRef] [PubMed]

19.

J. T. Motz, D. Yelin, B. J. Vakoc, B. E. Bouma, and G. J. Tearney, “Spectral- and frequency-encoded fluorescence imaging,” Opt. Lett. 30(20), 2760–2762 (2005). [CrossRef] [PubMed]

20.

D. Yelin, B. E. Bouma, S. H. Yun, and G. J. Tearney, “Double-clad fiber for endoscopy,” Opt. Lett. 29(20), 2408–2410 (2004). [CrossRef] [PubMed]

21.

M. Merman, A. Abramov, and D. Yelin, “Theoretical analysis of spectrally encoded endoscopy,” Opt. Express 17(26), 24045–24059 (2009). [CrossRef]

22.

K. K. Tsia, K. Goda, D. Capewell, and B. Jalali, “Simultaneous mechanical-scan-free confocal microscopy and laser microsurgery,” Opt. Lett. 34(14), 2099–2101 (2009). [CrossRef] [PubMed]

OCIS Codes
(050.1970) Diffraction and gratings : Diffractive optics
(110.2350) Imaging systems : Fiber optics imaging
(170.0110) Medical optics and biotechnology : Imaging systems
(170.2150) Medical optics and biotechnology : Endoscopic imaging
(260.2510) Physical optics : Fluorescence
(170.2655) Medical optics and biotechnology : Functional monitoring and imaging

ToC Category:
Medical Optics and Biotechnology

History
Original Manuscript: February 8, 2010
Revised Manuscript: April 11, 2010
Manuscript Accepted: April 12, 2010
Published: June 25, 2010

Virtual Issues
Vol. 5, Iss. 11 Virtual Journal for Biomedical Optics

Citation
Avraham Abramov, Limor Minai, and Dvir Yelin, "Multiple-channel spectrally encoded imaging," Opt. Express 18, 14745-14751 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-14-14745


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References

  1. B. I. Hirschowitz, “Endoscopic examination of the stomach and duodenal cap with the fiberscope,” Lancet Infect. Dis. 1, 1074–1078 (1961).
  2. C. Liang, K. B. Sung, R. R. Richards-Kortum, and M. R. Descour, “Design of a high-numerical-aperture miniature microscope objective for an endoscopic fiber confocal reflectance microscope,” Appl. Opt. 41(22), 4603–4610 (2002). [CrossRef] [PubMed]
  3. A. F. Gmitro and D. Aziz, “Confocal microscopy through a fiber-optic imaging bundle,” Opt. Lett. 18(8), 565–567 (1993). [CrossRef] [PubMed]
  4. M. A. D’Hallewin, S. El Khatib, A. Leroux, L. Bezdetnaya, and F. Guillemin, “Endoscopic confocal fluorescence microscopy of normal and tumor bearing rat bladder,” J. Urol. 174(2), 736–740 (2005). [CrossRef] [PubMed]
  5. V. R. Jacobs, M. Kiechle, B. Plattner, T. Fischer, and S. Paepke, “Breast ductoscopy with a 0.55-mm mini-endoscope for direct visualization of intraductal lesions,” J. Minim. Invasive Gynecol. 12(4), 359–364 (2005). [CrossRef] [PubMed]
  6. E. R. Sauter, H. Ehya, L. Schlatter, and B. MacGibbon, “Ductoscopic cytology to detect breast cancer,” Cancer J. 10(1), 33–41, discussion 15–16 (2004). [CrossRef] [PubMed]
  7. E. A. Reece, “Embryoscopy and early prenatal diagnosis,” Obstet. Gynecol. Clin. North Am. 24, 111–121 (1997). [CrossRef] [PubMed]
  8. A. J. Saltzman and S. Waxman, “Angioscopy and ischemic heart disease,” Curr. Opin. Cardiol. 17(6), 633–637 (2002). [CrossRef] [PubMed]
  9. C. M. Brown, P. G. Reinhall, S. Karasawa, and E. J. Seibel, “Optomechanical design and fabrication of resonant microscanners for a scanning fiber endoscope,” Opt. Eng. 45(4), 043001 (2006). [CrossRef]
  10. D. L. Dickensheets and G. S. Kino, “Micromachined scanning confocal optical microscope,” Opt. Lett. 21(10), 764–766 (1996). [CrossRef] [PubMed]
  11. Y. C. Wu, Y. X. Leng, J. F. Xi, and X. D. Li, “Scanning all-fiber-optic endomicroscopy system for 3D nonlinear optical imaging of biological tissues,” Opt. Express 17(10), 7907–7915 (2009). [CrossRef] [PubMed]
  12. G. J. Tearney, R. H. Webb, and B. E. Bouma, “Spectrally encoded confocal microscopy,” Opt. Lett. 23(15), 1152–1154 (1998). [CrossRef]
  13. D. Yelin, C. Boudoux, B. E. Bouma, and G. J. Tearney, “Large area confocal microscopy,” Opt. Lett. 32(9), 1102–1104 (2007). [CrossRef] [PubMed]
  14. C. Boudoux, S. H. Yun, W. Y. Oh, W. M. White, N. V. Iftimia, M. Shishkov, B. E. Bouma, and G. J. Tearney, “Rapid wavelength-swept spectrally encoded confocal microscopy,” Opt. Express 13(20), 8214–8221 (2005). [CrossRef] [PubMed]
  15. K. Goda, K. K. Tsia, and B. Jalali, “Serial time-encoded amplified imaging for real-time observation of fast dynamic phenomena,” Nature 458(7242), 1145–1149 (2009). [CrossRef] [PubMed]
  16. L. Froehly, S. N. Martin, T. Lasser, C. Depeursinge, and F. Lang, “Multiplexed 3D imaging using wavelength encoded spectral interferometry: a proof of principle,” Opt. Commun. 222(1-6), 127–136 (2003). [CrossRef]
  17. D. Yelin, I. Rizvi, W. M. White, J. T. Motz, T. Hasan, B. E. Bouma, and G. J. Tearney, “Three-dimensional miniature endoscopy,” Nature 443(7113), 765 (2006). [CrossRef] [PubMed]
  18. D. Yelin, W. M. White, J. T. Motz, S. H. Yun, B. E. Bouma, and G. J. Tearney, “Spectral-domain spectrally-encoded endoscopy,” Opt. Express 15(5), 2432–2444 (2007). [CrossRef] [PubMed]
  19. J. T. Motz, D. Yelin, B. J. Vakoc, B. E. Bouma, and G. J. Tearney, “Spectral- and frequency-encoded fluorescence imaging,” Opt. Lett. 30(20), 2760–2762 (2005). [CrossRef] [PubMed]
  20. D. Yelin, B. E. Bouma, S. H. Yun, and G. J. Tearney, “Double-clad fiber for endoscopy,” Opt. Lett. 29(20), 2408–2410 (2004). [CrossRef] [PubMed]
  21. M. Merman, A. Abramov, and D. Yelin, “Theoretical analysis of spectrally encoded endoscopy,” Opt. Express 17(26), 24045–24059 (2009). [CrossRef]
  22. K. K. Tsia, K. Goda, D. Capewell, and B. Jalali, “Simultaneous mechanical-scan-free confocal microscopy and laser microsurgery,” Opt. Lett. 34(14), 2099–2101 (2009). [CrossRef] [PubMed]

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