Fiber directional couplers made of highly asymmetric twin-cores (ATC), one of which is doped with erbium, are
designed to achieve an inherently gain flattened erbium doped fiber amplifier (EDFA). The refractive index profiles of
the fibers as well as the spacing between the two cores were carefully designed to achieve a targeted gain with low gain
excursions across the C-band. One of the designs yielded a theoretical median gain ~ 38 dB with an excursion within ±1
dB. In order to suite fabrication of such an inherently gain flattened EDFA by the MCVD fiber preform fabrication
process, the design had to be modified and a more modest target of about 20 dB was set with excursion below ± 1.5 dB
for metro-centric applications. It involved preparation of two independent preforms, which required selective polishing
of the cladding from one side by a certain amount to meet the required nominal separation between the two cores set at
the design stage of the fiber. Several intricate operations were required to implement the fiber drawing step from the two
assembled preforms. Preliminary characterization of the fabricated fiber shows filtering of ASE peak through selective
wavelength coupling from Er-doped core to un-doped core.
We present a high sensitive temperature sensor based on a side-polished fiber (SPF) coupled to a tapered multimode
overlay waveguide (MMOW). We have theoretically shown that the longitudinal tapering of the MMOW can be used to
tune the desired wavelength range in the spectrum without any loss in the sensitivity.
We investigate the inherent gain flattening characteristics of an EDFA based on a highly asymmetric dual-core photonic
crystal fiber for operation in the C-band. The gain flattening was achieved by exploiting the strong optical power
coupling between the two cores like that in a directional coupler at the phase matching wavelength (λ <sub>Ρ</sub>), which is
designed to be around 1533 nm. The inner core is partially doped with erbium. The fiber refractive index profile
parameters were so tailored such that a large fraction of the composite guided power flips from the un-doped outer core
to the inner erbium-doped core at wavelengths greater than λ <sub>Ρ</sub>. Thus the guided power at relatively longer wavelengths
gets amplified more as compared to that at shorter wavelengths in the C-band. This phenomenon resulted in an effective
flattening of the gain spectrum. Optimization of the design has led to an estimated median gain of ~ 21.2 dB with gain
excursion within ± 1.25 dB within the C-band (1532-1562 nm). Results of this work should be of importance for
realizing relatively inexpensive (due to cost saving on gain flattening filter head) and efficient EDFAs suitable for
potential deployment in transparent wide area and metro networks.
A high sensitive temperature sensor based on evanescent field coupling between a side-polished fiber half-coupler
(SPFHC) and a thermo-optic multimode overlay waveguide (MMOW) is designed and demonstrated. Such a structure
essentially functions as an asymmetric directional coupler with a band-stop characteristic attributable to the wavelengthdependent
resonant coupling between the mode of the SPFHC and one or more modes of the MMOW. A slight change in
temperature leads to a significant shift in the phase resonance-coupling wavelength ( λ<sub>r</sub> ) between the MMOW and
SPFHC λ<sub>r</sub>, which is easily measurable. The wavelength sensitivity of the device is measured to be ~ 5.3 nm/°C within
the measurement range of 26-70°C; this sensitivity is more than 5 times higher compared to earlier reported temperature
sensors of this kind. The SPFHC was fabricated by selective polishing of the cladding from one side of a bent
telecommunication standard single-mode fiber and the MMOW was formed on top of the SPFHC through spin coating.
A semi- numerical rigorous normal mode analysis was employed at the design stage by including the curvature effect of
the fiber lay in the half-coupler block and the resultant
<i>z</i>-dependent evanescent coupling mechanism. An excellent
agreement between theoretical and experimental results is found.