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Introduction
As optical
fiber imaging technology such as Optical Coherence
Tomography (OCT) and microconfocal technology advances, the
need for single mode optical fiber probes with confocal
lens also raises. Ball lens can increase the lateral
resolution and increase the NA of the fiber. However, in
order to focus light from an optical fiber through a small
ball lens, a distance between the tip of the optical fiber
and the lens is needed for the light rays to expand. The
solution of this problem can be dealt with by placing a
spacer between the tips of the optical fiber and the ball
lens. Doing so often increase the size of the fiber probe or
adding complexity to the probe. Looking into the future as
OCT and microconfocal advances into the medical imaging
fields such as imaging eye retina and brain blood cells,
disposable probes with ball lens having the same diameter of
a signle mode fiber without adding structure complication to
the probe would be ideal. Therefore, a proof of concept way
of making optical fiber probes with focus lens at the tip is
studied.
In the
paper, we introduce a novel growing technique for forming
confocal lens at the tip of optical fiber probe while
allowing necessary space between the ball lens and fiber end
for the light ray to expand. In this study, we applied the
Norland 81 (NOA81) optical glue multiple times to the end of
optical fiber. This growing process is been observed and
controlled under microscope to reach optima dome forming.
The probe then is being measured, and simulated by both ray
tracing and Zemax. Finally, the probe is connected to a
common path OCT system to verify the simulations results and
images of layer of tape and eye retina us taken. A bare
fiber probe is used for comparison.
Growing technique and probe design.
Optical adhesive Noralnd 81 (NOA 81) is first applied to the
tip of an optical fiber pointing vertically downward, and a
small dome is formed due to gravity, then the newly formed
dome is cured by a UV light. The same process is preceded
for multiple times until the ideal ball lens structure is
formed. Each time the process is being applied, the size of
the dome grows, so is the spacing between the fiber end and
the lens, allowing light rays to expand. For the specific
design of an OCT probe built for imaging the layers eye
retina. Therefore, a fiber probe with the OD of 124um is
desired to fit it into eye surgical tools. Moreover, a
working space of 1mm is desired. According to Matrix ray
tracing, we have the following design: The ball lens is
formed on a single mode fiber working at 800nm, has a
core/clad diameter of 5.25/124um. The O.D. Of the protecting
layer of the fiber is 245um. The radius of the ball lens is
measured to be 175um, and the length expanding region is 180
um. The characteristic of the simulated results is presented
as the curves of the beam radius vs. axial distance in
Figure 2. From the figure, we find that the lens has a focus
point at 1mm. The same characteristic curve of a bare fiber
is also given in the figure for comparison.
Fig.1
Fig.2
Fig.3 Fig.4
Conclusion
In this paper, we have demonstrated a
novel technique of forming confocal lens at the tip of a
single mode bare fiber. Our simulated results concurs
with our experimental data. Further research should be
done to make the lens more uniform in order to reduce
interference between layers, and making the dome more
robust. Applying a gold layer onto the lens can be
beneficial by increasing the returned reference signal
and protecting the lens.
References
[1] Chris W. Barnard and John W.Y. Lit,
“single mode fiber microlens with controllable spot
size” Applied Optics/Vol. 30 No. 15/20 May 1991
[2] Enbang Li, G. D. Peng, and X. Ding,
“High spatial resolution fiber-optic fizeau
interferometric strain sensor based on an in-fiber
spherical microcavity” Applied Physics Letters 92,
101117(2008)
[3] Donald J. Hayes and T. Chen, “Next
generation optoelectronic components enabled by direct
write microprinting technology” Proc. Of SPIE Defense
and Security Symposium, April 2004.
[4] Ik-Bu Sohn, Y. l Noh, and J. Lee,
“Side-imaging lensed photonic crystal fiber probe” Porc.
of SPIE Vol.6847 68472-1
[5] X. Liu, X. Li, D. Kim2, I. Ilev, J.
U. Kang, “Fiber-optic Fourier-domain common-path OCT”,
Chinese Optics Letters, 6, (2008).
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