Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 267

Fig. 8.6 ( a ) Conceptual drawing of a piezo-driven single-mode fiber scanner miniature endoscope.
( b ) Picture of a resonating device for endoscopic application [ 38 ]. Note that diameter of the piezo
component including mounting base is 1.8 mm
L and R are respectively the length and radius of extended fiber. E and denote
Young's modulus and the mass density. ˇ is a constant related to the vibration
mode and boundary conditions of the fiber fixture. It is also possible to design a
scanning mechanism with a short scanning range while optically relaying the change
of angle of the fiber tip. In this case, a small lens is attached at the distal end of the
probe. A comparison of both cases is demonstrated in Fig. 8.7 . The detailed design
principles are provided in [ 19 ].
The second technique is MEMS technology to fabricate miniaturized beam
scanning systems [ 41 ] which can be mounted distal to a single, fixed optical fiber
which both illuminates and collects light from the sample. Miniaturized optics
are then located distal to the MEMS component to focus the scanning beam onto
the tissue and collect remitted light. Several techniques for fabricating MEMS
scanners have been reported from early work by Dickensheets and Kino [ 42 ], who
fabricated a scanner comprising two separate mirrors with orthogonal scan axes
by micromachining and etching in silicon. Solgaard's group at Stanford designed
a scanner with a single reflecting mirror in a two-axis gimbaled arrangement
which eliminated the need to incorporate a pair of discrete scanners to achieve a
two-dimensional scan [ 43 ]. This design was incorporated in a benchtop prototype
confocal microscope which uses a single polarization-maintaining fiber to transmit
light to and from the tissue sample with a compact 5-mm-diameter 3-element
objective lens positioned with its rear focal plane coincident with the MEMS
scanner [ 23 ]. The system demonstrated reflectance confocal imaging of ex vivo and
in vivo human oral mucosa at 8 frames per second, resolving individual cell nuclei.
However, the FOV was relatively small, at only 140 70 m, due to the maximum
achievable scan angles provided by the MEMS mirror. The same fabrication process
involving deep reactive ion etching was used to fabricate a different MEMS scanner
design, with two separate reflecting regions each to accommodate separate beams,