materials that transmit light from ultra-violet to mid-IR without any absorption
peak. Commercial HMFGfibers include InF 3 (with attenuations of <0.25 dB/m
between 1.8 and 4.7μm) and ZrF 4 (with attenuations of <0.25 dB/m between 1.8
and 4.7μm). Such optical losses allow for practical short and medium length
applications of a few meters. The reliability of HMFGfibers depends on protecting
thefiber from moisture and on manufacturing schemes to reduce surface crystal-
lization. Thus HMFGfibers are a good choice for the 1.5-to-4.0-μm spectral region,
which is increasingly used in medical applications.
Chalcogenide glassesare based on the chalcogen elements sulfur (S), selenium
(Se), and tellurium (Te) together with the addition of other elements such as ger-
manium (Ge), arsenic (As), and antimony (Sb) [ 5 ]. These glasses are very stable,
durable, and insensitive to moisture. Chalcogenidefibers are able to transmit longer
wavelengths in the IR thanfluoride glassfibers. Longer wavelengths are transmitted
through the addition of heavier elements. All chalcogenidefibers have strong
extrinsic absorption resulting from contaminants such as hydrogen. Although the
losses of chalcogenides are generally higher than thefluoride glasses, these losses
are still adequate for about 2-m transmissions in the spectral band ranging from 1.5
to 10.0-μm. Some typical loss values are less than 0.1 dB/m at the widely used 2.7-
μm and 4.8-μm wavelengths. The maximum losses for the three common chalco-
genides are as follows:
(a) Sulfides: <1 dB/m over 2 to 6μm
(b) Selenides: <2 dB/m over 5 to 10μm
(c) Tellurides: <2 dB/m over 5.5 to 10μm
Applications of chalcogenide fibers include Er:YAG (2.94μm) and CO 2
(10.6μm) laser power delivery, microscopy and spectroscopy analyses, and
chemical sensing.
Germanate glass(GeO 2 ) fibers generally contain heavy metal oxides (for
example, PbO, Na 2 O, and La 2 O 3 ) to shift the IR absorption edge to longer wave-
lengths [ 5 ]. The advantage of germanatefibers over HMFGfibers is that GeO 2 glass
has a higher laser damage threshold and a higher glass transition temperature, thus
making germanate glassfibers more mechanically and thermally stable than, for
example, tellurite glasses. GeO 2 glass has an attenuation of less than 1 dB/m in the
spectrum ranging from 1.0 to 3.5μm. A key application of germanatefibers is for
laser power delivery from the laser to a patient from Ho:YAG (2.12μm) or Er:
YAG (2.94μm) lasers, where thefiber losses nominally are 0.25 and 0.75 dB/m,
respectively. Thesefibers can handle up to 20 watts of power for medical proce-
dures in dermatology, dentistry, ophthalmology, orthopedics, and general surgery.
Crystalline IRfiberscan transmit light at longer wavelengths than IR glass
fibers, for example, transmission up to 18μm is possible [ 5 ]. Thesefibers can be
classified into single-crystal and polycrystallinefiber types. Although there are
many varieties of halide crystals with excellent IR transmission features, only a few
have been fabricated into opticalfibers because most of the materials do not meet
the required physical property specifications needed to make a durablefiber.
82 3 Optical Fibers for Biophotonics Applications