For a suspended nanowire, the holes surrounding the core are expected to be as large as possible to propagate the light at
wavelengths as long as possible. However, the fabrication of nanowire surrounded with large holes is still a challenge so
far. In this paper, a method which involves pumping positive pressure of nitrogen gas in both the cane fabrication and
fiber-drawing processes, is proposed. A suspended nanowire, with a core diameter of 480 nm and an unprecedented large
diameter ratio of holey region to core (DRHC) of at least 62, is fabricated in the length of several hundred meters. Owing
to the large holes, the confinement loss of the suspended nanowire is insignificant when the wavelength of light
propagated in it is 1700 nm. Additionally, the tube-shaped glass cladding of the suspended nanowire shifts the singlemode
cutoff wavelength to 810 nm, which is much shorter than the cutoff wavelength, 1070 nm, of a naked nanowire
with the same diameter. A single-mode supercontinuum (SC) generation covering a wavelength range of 900-1600 nm is
obtained under 1064 nm pump pulse with the peak power as low as 24 W. A single-mode third harmonic generation
(THG) is observed by this nanowire under the pump of a 1557 nm femtosecond fiber laser. This work indicates that the
suspended nanowire with large holes can provide high nonlinearity together with single-mode propagation, which leads
to interesting applications in compact nonlinear devices.
Supercontinuum (SC) generation has the important applications such as broadband light source, optical coherence
tomography, ultra-short pulse compression, and optical frequency metrology, etc. Tellurite glass is transparent in the
mid-infrared range, and has a higher n2 than silica glass by at least one order of magnitude. We have fabricated the
hexagonally shaped tellurite air-clad fiber with a core diameter of around 1 μm through controlling the temperature field
exactly in the process of fiber-drawing. Since the SC generation strongly depends on the chromatic dispersion, which is
determined by the microstructure of fiber, it is interesting to investigate and demonstrate such dependence for such a
small core fiber in detail. In this work by pumping a positive pressure of nitrogen gas into the holes of preform, we
obtained 1 μm core fibers with diameter ratio of holey region to core (DRHC) varied from 3.5 to 20. The dispersion was
tailored effectively by the variation of DRHC. Dependences of SC on the microstructure and dispersion were
demonstrated. The pump lasers were picosecond and femtosecond fiber lasers. One octave flattened SC generation was
obtained for the fibers pumped by 1064 nm picosecond fiber laser with the pulse energy of several hundred pJ. Intense
second and third harmonic generations were obtained under the pump of a 1557 nm femtosecond fiber laser. The
correlation of dispersion and SC spectra was analyzed. Such tellurite microstructured optical fibers (MOFs) with high
nonlinearity and controlled dispersion are significant in nonlinear applications.
We present the simulation results of the optical parametric amplification in the composite chalcogenide-tellurite
microstructured fiber driven by a single wavelength. Further, by applying dual pumping scheme, with two equal power
pumps with different wavelengths, lying on opposite sides of the zero dispersion wavelength (ZDWL), we achieve a
relatively flat gain spectrum over a wider bandwidth than that possible for single pump. The two pumps in the dual
pumping scheme have the power half that of the pump power in single pumping scheme. The composite microstructured
fiber designed here not only shows zero dispersion in the telecommunication band but also has two ZDWLs (one in the
telecommunication band at 1.51 μm and the other at 2.19 μm) with anomalous dispersion between the two ZDWLs. In
addition, the composite fiber has high nonlinearity (of the order of 16 W-1m-1). With a single pump at the first ZDWL,
1.51 μm, the parametric gain over more than 1000 nm wide wavelength band, starting from 1.14 to 2.21 μm, is achieved
with a 11.13 dB gain difference between the gains near the pump and optimal wavelengths. In the dual pumping scheme,
the difference between the gains at optimal wavelengths and pump wavelength is only 2.45 dB; while the difference
between the maximum and minimum gain is 3.43 dB. The maximum value of the gain at the optimal wavelengths, in
both single pumping and dual pumping schemes are same. We further show that by selecting proper pump wavelengths
ultra-broadband gain can be achieved.
Tellurite highly nonlinear microstructured fibers were fabricated by pumping a positive pressure of nitrogen gas into the
holes of cane in the fiber drawing process. By adjusting the pump pressure to inflate the holes of the fiber, the
microstructures were reshaped, and the chromatic dispersions were tailored. Two kinds of fiber were fabricated. One is
an air-clad fiber with a 1 μm hexagonal core, which is the smallest core in this shape for the air-clad fiber. By changing
the inflation pressure, the diameter ratio of holey region to core (DRHC) was changed in the range of 1-20. Fibers with
DRHC of 3.5, 10, 20 were demonstrated. By increasing the DRHC, the zero dispersion wavelengths were shifted to the
short wavelength and the confinement loss were decreased. Another is a complex microstructure fiber with a 1.8 μm core
surrounded by four ring holes. The shape of the microstructure was reshaped so heavily by the inflation pressure that it is
obviously different from the original shape in the cane. The correlations among pump pressure, hole size, surface tension
and temperature gradient were investigated. The temperature gradient at the bottom of the preform's neck region was
evaluated quantitatively. The chromatic dispersion of this fiber was compared with that of a step-index air-clad fiber. It
was found that this fiber had a much more flattened chromatic dispersion. Supercontinuum generations were investigated
by the pump of a 1557 nm femtosecond fiber laser. Intense third harmonic generations were obtained from the 1μm
haxgonal core fiber. Broad and flattened spectrum was obtained from the complex microstructure fiber. This
investigations show that, by using a positive pressure to reshape the microstructure and by controlling the fabrication
conditions exactly, highly nonlinear soft glass fibers with desirable chromatic dispersion can be fabricated, and such
fibers have interesting applications in highly nonlinear field such as THG and SC generation.
We propose a chalcogenide (As2S3) core tellurite cladding microstructured fiber with
flattened normal dispersion for ultraflat supercontinuum (SC) generation. To realize flattened
normal dispersion, the structure parameters are optimized such as the chalcogenide core diameter,
the air hole diameter and the distance between the centers of the two neighboring air holes. The
ultraflat normal dispersion curve is obtained and the pulse propagation is investigated using a
nonlinear Schrödinger equation. It is shown that an ultraflat SC spectrum with deviations less
than 4 dB over an octave (from 1400 nm to 3000 nm) can be achieved by the illumination of a
pulsed light with a pulse width of 200 fs, central wavelength of 2000 nm and peak power of 1000
W.
We present the design, dispersion calculations and tailoring, and simulation of the parametric gain in the composite
tellurite-fluorophosphate glass fiber. The fiber has the tellurite core subsequently surrounded by fluorophosphate and
tellurite claddings. Thermal properties of the tellurite and fluorophosphate glasses match, depicting the feasibility of the
fabrication of the fiber under controlled environment. The composite fiber introduced here has the advantage of easy
handling over the fiber tapers or the air cladding tellurite nanofibers. With our analysis we observe that the
fluorophosphate and the tellurite cladding thickness along-with the fiber core diameter, have control on the dispersion
and the parametric gain. The wavelength band, over which the dispersion is anomalous, increases with increasing
tellurite core diameter. For longer wavelengths, increasing the fluorophosphate cladding thickness causes increase in the
anomalous dispersion bandwidth and further flattening of the dispersion curve. The slope of the dispersion curve near the
zero dispersion wavelength (ZDWL) is greatly reduced for thicker outer tellurite ring claddings. It is possible to design
zero flattened dispersion fiber or the fiber providing two ZDWLs in the communication band, which can generate
broadband parametric amplification. The gain obtained has strong bandwidth dependence on the dispersion slope. We
study the effects of the fiber length, pump power, and pump detuning from the ZDWL, on the parametric amplification.
With multiple pumping with proper selection of the pump wavelengths, the parametric amplification process can
generate ultra flat, broadband amplifiers as the dispersion provided by the composite fiber is anomalous over a wide
bandwidth.
We present the design of the chalcogenide (As2S3) glass nanofibers with the nonlinear tellurite glass as the cladding
material. Both of these glasses have high nonlinearity. We show that the simple step index structure with tellurite
cladding has the normal dispersion in the telecommunication window for any value of the core diameter, ranging from
the sub-wavelength size to a few micro-meters. The dispersion achieved is flat so as to be applicable to dispersion
compensating devices. However, the dispersion can not be tailored to zero. So we propose the photonic crystal fiber
(PCF) structure, which has six circular identical air holes introduced in the tellurite cladding around the chalcogenide
core. The air holes introduced cause the decrease in the effective cladding index. This structure shows anomalous
dispersion in the telecommunication band with two zero dispersion wavelengths. With further optimization the structure
can show zero flattened dispersion. We study the effect of the various design parameters - the chalcogenide core
diameter, the air hole diameter, and the pitch, that is the distance between the centers of two neighbouring air holes,
which is same as the distance between the center of the core and any of the air holes. We optimize the design to achieve
zero slope (flattened) at the zero dispersion wavelength, 1.55 μm. The thermal characteristics of tellurite glass match
with the chalcogenide making the fabrication of the chalcogenide core PCF with the tellurite glass as a cladding material
feasible. We also present the calculation of the nonlinearity of the PCF.
We design air cladding tellurite (TeO2), bismuth oxide (Bi2O3) based, and chalcogenide (As2S3) nanofibers, and calculate
the chromatic dispersions. For each material, wavelength dependent propagation constants of the nanofiber are obtained
from the exact solutions of the Maxwell's equations, and from the propagation constants the chromatic dispersion is
calculated. We tailor the dispersion to zero at the communication wavelength, 1.5 μm, by proper selection of the core
diameter of the nanofiber for all the above materials. We further explain the technique for flattening the zero dispersion
in telecommunication window, using glass instead of air, as the cladding of the nanofiber structure. Using the glass
cladding has the advantage of easy handling, specially, for the communication purposes. Further, the glass cladding
causes larger effective index difference between various modes of the nanofiber, thus reducing the mode coupling. We
present the numerical results of the dispersion flattening technique by assuming the borosilicate glass cladding to the
chalcogenide As2S3 glass core nanofiber. With the borosilicate cladding the dispersion characteristics of the nanofiber
change drastically and flattening of the zero dispersion is achieved at 1.408 μm wavelength, when the core diameter is
724 nm.
The principle of operation and the design techniques for various types of the optical demultiplexers are reviewed. The results for two-channel optical demultiplexer based on optical directional coupler configuration are presented. The device for separating two wavelengths with 4 nm spacing is designed on the Silicon substrate in SiO2/SiO2-TiO2/SiO2 rib waveguide. The rib waveguide design techniques using the effective index method are described. The propagation of the two wavelengths through the device is calculated using the computer tools based on the beam propagation method (bpm). The dependence of the device length required for separation of the two wavelengths on the rib waveguide parameters is studied. The waveguide rib width and separation between the parallel waveguides are the parameters those affect the value of the length required for separating the two wavelengths to a greater extent. The guide thickness inside the rib also has considerable effect on the device length. The clad thickness and rib height, however, were found to be the least affecting parameters. The optimization techniques for the rib waveguide and the coupler parameters for achieving the demultiplexing with the minimum length of the device are explained. The proposed fabrication techniques for the SiO2/SiO2-TiO2/SiO2 rib waveguide are presented.
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