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Optical modeling of an ultrathin scanning fiber endoscope, a preliminary study of confocal versus non-confocal detection

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Abstract

An optical model of an ultrathin scanning fiber endoscope was constructed using a non-sequential ray tracing program and used to study the relationship between fiber deflection and collection efficiency from tissue. The problem of low collection efficiency of confocal detection through the scanned single-mode optical fiber was compared to non-confocal cladding detection. Collection efficiency is 40x greater in the non-confocal versus the confocal geometry due to the majority of rays incident on the core being outside the numerical aperture. Across scan angles of 0 to 30°, collection efficiency decreases from 14.4% to 6.3% for the non-confocal design compared to 0.34% to 0.10% for the confocal design. Non-confocality provides higher and more uniform collection efficiencies at larger scan angles while sacrificing the confocal spatial filter.

© 2005 Optical Society of America

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Figures (9)

Fig. 1.
Fig. 1. Fiber microscanner assembly contained in a housing of one millimeter outer diameter.
Fig. 2.
Fig. 2. SFE image of 1951 USAF test target
Fig. 3.
Fig. 3. A plot of the scatter model function in ASAP 8.0.3, based on the above H-G’ function (2) with ϕo = 25 degrees, a = 17.25*π, and β = 0.58[9].
Fig. 4.
Fig. 4. (above). A schematic showing the overall system and the relationships between various parameters. The fiber tip displacement to inner edge of housing is exaggerated for clearly illustrating scan angle. Rays shown are representations only, and do not represent energy traveling along actual ray paths. (below). The optical fiber end face is shown with illustrative cones of acceptance angles (NA) denoting light collection in confocal and non-confocal. The optical fiber end face is shown as the detector with illustrative cones of acceptance angles (NA) denoting light collection in confocal and non-confocal geometries. The vertex of the two cones is located at the effective point source of the single-mode optical fiber, slightly within the endface surface along the optical axis.
Fig. 5.
Fig. 5. The plots above show a profile of the system tracing rays at theta values of 0, 1, 2 and 3, and the corresponding location of the energy distribution on the sample. The scanning optical fiber is shown in Green. The vertical arrow indicates the location of the (ideal) lens.
Fig. 6.
Fig. 6. (a) (Left). shows the spot from a global perspective. (b) (Right) shows the same spot centered in the picture window. The majority of the sampled rays are distributed within a 2 micron spot size.
Fig. 7.
Fig. 7. Intensity of the rays scattering off of the sample at θ=2 degrees plotted ± 90 degrees on both axes.
Fig. 8.
Fig. 8. Energy incident on an oversized 6-mm diameter detector at a reflection angle of 20 degrees (θ=2) that slightly overfills the square window. Note the specular component on the left region of the detector, and the “hole” in the detector is where the microscanner housing and lens interface the surface of the much larger detector.
Fig. 9.
Fig. 9. Flux on internal components as a function of rotation angle. Note that the dashed lines combine to give the solid Cyan colored line.

Tables (1)

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Table 1. Images of energy density on non-confocal and confocal detection systems

Equations (2)

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H G = 1 4 π 1 g 2 ( 1 g 2 2 g cos ϕ ) 3 / 2
H G = a * β cos ( ϕ ) ( 1 a ) + ( 1 β ) 1 g 2 ( 1 g 2 2 g cos ( ϕ ϕ o ) ) 3 / 2
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