OH content for operation in the UV, visible, and near-IR regions. The mechanical,
optical, and structural properties of HCSfibers are especially useful in applications
such as laser delivery, endoscopy, photodynamic therapy, and biosensing systems.
3.8 Coated Hollow-Core Fibers
Because a conventional solid-core silicafiber exhibits tremendously high material
absorption above about 2μm, internally coated hollow-corefibers provide one alter-
native solution for the delivery of mid-infrared (2– 10 μm) light to a localized site [ 5 ].
Section3.12describes middle-infraredfibers as another alternative for operation above
2 μm. Examples of light sources operating above 2μmincludeCO 2 (10.6μm) and Er:
YAG lasers (2.94μm), which have wide applications in urology, dentistry, otorhino-
laryngology, and cosmetic surgery. In addition, hollow-corefibers are useful in the
ultraviolet region, where silica-basedfibers also exhibit high transmission losses.
As shown in Fig.3.12, a hollow-corefiber is composed of a glass tube with
metal and dielectric layers deposited at the inner surface plus a protection jacket on
the outside [ 26 – 29 ]. In the fabrication process of thesefibers, a layer of silver
(Ag) is deposited on the inside of a glass tube, which then is covered with a thin
dielectricfilm such as silver iodide (AgI) [ 5 ]. Light is transmitted along thefiber
through mirror-type reflections from this inner metallic layer. The thickness of the
dielectric layer (normally less than 1μm) is selected to give a high reflectivity at a
particular infrared wavelength or a band of wavelengths. Tubes of other materials,
such as plastic and metal also are employed as hollow-core waveguides, but glass
hollow-corefiber provides moreflexibility and better performance. The bore sizes
(diameters of thefiber hole) can range from 50 to 1200μm. However, since the loss
of all hollow-corefibers varies as 1/r^3 , where r is the bore radius, the moreflexible
smaller bore hollow-corefibers have higher losses.
Compared to solid-corefibers, the optical damage threshold is higher in
hollow-corefibers, there are no cladding modes, and there is no requirement for
angle cleaving or anti-reflection coating at thefiber end to minimize laser feedback
effects [ 5 ]. To direct the light projecting from the hollow-corefiber into a certain
direction, sealing caps with different shapes (e.g., cone and slanted-end) at the distal
AgI ( 0.5 μm)Glass capillary tubeProtection jacketHollow air core
(200-1000 μm)Fig. 3.12 The cross-section
of a typical hollow-corefiber
3.7 Hard-Clad Silica Fibers 77