Endomicroscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 265

Fig. 8.4 Schematic of the single-mode index fiber

Fig. 8.5 Photographs of selected PCFs applicable to nonlinear endomicroscopy. a large-mode-
area PCF, b hollow-core photonic bandgap fiber, and c double-clad PCFs [ 21 ]

Specialized fibers with tailored dispersion properties have been developed which
can deliver the required laser pulses over the distances required for endomicroscopy
with minimal dispersion or distortion. Photonic bandgap fibers and photonic crystal
fibers (PCF) confine and guide light by use of periodic arrays of air holes. The core
of the fiber is either made of air (which does not introduce SPM) or has a large mode
area which allows delivery of light at lower intensity, reducing the generation of
unwanted nonlinear effects in the delivery fiber. A third type of fiber for nonlinear
endomicroscopy is the double-clad fiber, comprising a single-mode core for light
delivery, surrounded by a larger multimode core for efficient light collection. Since
all light generated in the tissue by nonlinear processes carries useful information, the
aim is to simply collect as much light as possible, without the need for single-mode
collection and descanning required by confocal imaging. Examples of selected PCF
types are shown in Fig. 8.5 .
Main characteristics of these fibers (Fig. 8.5 ) include [ 21 ]:

- Large-mode-area PCF provides a low level of self-phase modulation and allows

single-mode operation over the broad pulse bandwidth. On the other hand, it has
low collection NA and requires dispersion compensation.

- Hollow-core photonic bandgap fiber works within narrow wavelength range and

has low effective collection NA. It is, however, very efficient and allows minimal
propagation loses. It also does not require prechirping compensation.

- Double-clad PCF is characterized with low self-phase modulation, has a high

collection NA (cladding), and provides dual illumination/collection functions.
Similar to large-mode-area PCF, it requires dispersion compensation.