are patented. A photonic crystal structure as an overlay on the side-polished
D-shape fiber is proposed [152]. The devices can be tunable if electro-optic active
materials are used. A method to fabricate electrodes on D-shape fibers has also
been proposed and patented [153].
Among the several methods reported to introduce rare earth dopant into the
fiber, the solution doping technique, developed by Townsend et al. [154] in 1987,
is the simplest and, therefore, the most popular. The MCVD process for the
germania core deposition is carried out at a reduced temperature so that a
partially sintered germania layer inside the quartz tube is formed. A solution
of salt of the rare earth (usually aqueous or alcoholic solutions of nitrate of
chloride salts) soaks into porous soot and then is glassed at a higher temperature.
The tube is then collapsed to form preform. Passing chlorine through the heated
tube before the glassification reduces the OH content considerably. The concen-
tration is limited by the clustering of the rare earth ions, which quench the photo-
excited ions. Co-doping with aluminum isolates the rare earth ions by forming a
salvation shell at each neodymium ion and allows for a high doping level of the
rare earth ions without clustering [155]. A 33-dB small signal gain was reported
on a 23-m long Al–Nd co-doped fiber under 50-mW pump power, whereas the
small signal gain was only 7 dB for the Nd-only–doped fiber under the same
pumping conditions [156].
Several milliwatts of superfluorescent light at approximately 1080 nm were
measured when a 9-m long fiber was forwardly pumped by an 820-nm LD that
delivers approximately 20-mW pump power into the fiber. A fiber optic gyro-
scope for finding true north was demonstrated at KVH using this superfluo-
rescent light source. Gain anisotropy was observed in a 300-ppm Nd-doped fiber
with core size of 2.5 by 1.25 mm and core-cladding index difference of 0.032.
Under the measurement conditions of 21-mW pump power at 810 nm and
450 mW seeding power at 1088 nm, the small signal gains were 3.4 and 3.1 dB,
respectively, when the polarization directions for the pump and seeding light are
in both the major and the minor axes [157–159]. The anisotropic behaviors of
the gains are consistent with a model based on stronger confinement of the odd
mode. Microsecond optical-optical switching in this type of fiber was
demonstrated [160]. In this device, the effective indices of a two-mode fiber
interferometer operated at 633 nm were resonantly modulated using an 807-nm
pump laser, which results in switching between the two interferential lopes.
Erbium- and ytterbium-doped PM fibers were fabricated and studied, mainly
motivated as a potential, but expensive, candidate to solve signal distortion
problem caused by the polarization dispersion [161]. A 14-dB net gain was
Mendez / Specialty Optical Fibers Handbook ch16 Final Proof page 553 25.10.2006 12:23pm
Rare Earth-Doped Elliptical Core Fiber 553

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