Phoyonic Crystal Fiber_Aziz Mahfoud
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Transcript of Phoyonic Crystal Fiber_Aziz Mahfoud
Photonic Crystal Fiber Aziz Mahfoud Introduction
In the last decade, a great deal of work has been focused in photonic crystals due mainly
to their optical properties. An interesting characteristic display by such structure is the
selectivity of the electromagnetic field that can propagated along them, in other words the
propagation of certain frequency is prohibited. Recently applications have been devise in
the fields of optical communication, high resolution spectral filters, photonic crystals film
(counterfeit for credit cards), photonic crystal lasers. A new type of waveguide has assure
to perform in very way that traditional core-cladding fiber. Photonic crystal fibers
(hollow fiber, see figure 1) promise to become the next generation of ultra-low loss
transmission fiber, their also have applications in power deliveries , sensors and nonlinear
optics. In commercial available holey fibers more than 95 percent of the light travels in
the hollow core or in the holes of the cladding. Due to the air-holes Fresnel reflection at
the end of the endfaces estimated less than 10-4 . Since up to 85 % of the fiber cross
section is composed of fused silica, fusion splicing become easier than conventional
fibers, and also good temperature stability of optical properties, along with low bending
losses. More recently scientists from blaze photonics have reported transmission losses of
about .58 dB/km (0.18 background Rayleigh scattering, 0.13 dB/km hydroxyl absorption,
0.27 dB/km excess loss associated with the geometry of the fiber.) for a solid core fiber
at 1.55 um which approach 0.2 dB/km the standard single mode optical fiber. In the same
context Corning, have reduced the losses in air-core photonic crystal fiber (PCF) from
1000 dB/km to 13 dB/km.
Figure 1. Hollow core photonic bandgap fiber. Diameter of the holey region about 40 um, diameter of the silica cladding about 135 um, with coating (acrylate) diameter of 220 um.
Distance between cladding hole enters 2.3 um, with a core diameter of 9.3 um.
Fabrication
Photonic crystal fibers are made by stacking tubes and rods of silica glass into a large
structure (perform) of the pattern of holes required in the final fiber. The perform is then
bound with tantalum wire and then is taken to a furnace of fiber drawing tower . The
furnace is filled with argon and reach temperature about 2000 oC as consequence the
glass rods and tubes get soften. Later, the preform is fused together and reduced to 1mm
size with hole around 0.05 mm diameter (see figure 2). In other words by increasing the
furnace temperature the air hole size can be reduced. After reducing the preform to a size
that is 20 times smaller the structure thus formed (cane), the hole process is repeated and
spaces between the holes of 25 millionth of meter can be obtained. Defects are created by
replacing tubes for solid rods (as in the case of highly nonlinear PCF or by removing a
group of tubes from the preform (hollow core photonic PCF). Since the fabrication
process is quite robust complicated geometries can be achieved. For instance the
geometry of the center defect can be modified by the introduction of the thicker of
thinner tubes at different position around the defect.
Figure 2. Typical fabrication process in a photonic crystal fiber. The tub and rods are gather together and a furnace is used to heat the preform to 2000 oC,.The preform then fused together reducing its original size with a typical ratio of about 20.. The right hand side figure shows the first photonic crystal fiber
Guiding process
It is important to describe the mechanism or mechanisms through what the light is
guided in a photonic crystal fiber. Since a PCF is a structure that usually is formed by a
center core surrounding by a periodic structure (cladding), basically we can think in two
possible guiding mechanism. First, we can consider the case in which the center core has
a effective index of refraction than the cladding. In this case a total internal refraction
kind of process occurs. We know that a ray optics picture, is not a accurate description of
the electromagnetic propagation of a wave, and a wave formalism is necessary for a
better understanding. The reality is that the fundamental mode is confined to the core due
to the higher index of refraction, and it is enable to leak away since the corresponding
electromagnetic field are not a mode (solution of Maxwell’s equation for this geometry).
In the other hand, higher order modes can be leak away of the core (a higher field
distribution in the cladding compare with the fundamental mode). An important
characteristic of PCF if that for small enough holes (air) the fiber remains single mode at
all wavelength, this types of fiber are know “endlessly single mode fiber”.
The other guidance mechanism is a novel one and it is based in a concept
that has attracted the attention of many researchers worldwide. Photonic band gap (PGB)
theory has opened a numerous application including PCF. PBG exhibit the characteristic
that only certain wavelength can be transmitted. So, in a PCF the guidance process is
achieved by coherent backscattering of the light into the core. Light incident upon the
core-cladding interface is strongly scattered by the air-holes. For particular wavelengths
the scattering process results in constructive interference of the reflected rays in the core.
That guidance process enable to fabricate fiber with hollow cores (filled with air)
something that it is impossible with conventional fiber (in a conventional fiber if the core
is hollow, it is impossible to obtained a material with lower index of refraction than air as
cladding)
Optical properties of 2-D photonic crystals
Since PCF are based on the physics of PBG, I found very important to provide a
introduction of PBG and the ways of it can control the propagation of the radiation of
electromagnetic waves. In a photonic bandgap structure there are certain frequency that can not
propagate along the direction of the periodic structure, and is this property that is exploited for
controlling the electromagnetic radiation. In essence the band calculations show the functionality
between wave vector and frequency. The relationship between frequency and wave vector is
usually very complex and numerical techniques are necessary (the plane wave method is a well
employ technique to calculate band structure). More general, we can think that there are certain
frequencies that can not propagate in the plane of the periodic structure. That properties is what is
exploited in a photonic crystal fiber, and thus it constitute the principle behind the guidance
process in the fiber. Another important characteristic that distinguish photonic crystal fiber from
regular fiber is the fact that the fiber can make to support one mode for all wavelength. This is
quite remarkable since usually regular fiber become multimode a shorter wavelength. It is
interesting to see, that also photonic crystal fiber are not bound for the size limitation that must be
imposed in a regular fiber in order to obtained single mod operation, that allow PCFs suitable for
high power application Figure 3 shows a typical 2-D band structure calculation.
Fig.3 calculated photonic band structures of a hexagonal lattice of dielectric cylinders in air E-polarization (solid lines) and H-polarization (dashed lines). The cylinders have a refractive index of 3.6.
Applications
Large mode area fiber
We already have mentioned in our introduction, that certain geometries will
support only one mode, no matter the size of the core or the wavelength. These types of
fiber are called endlessly single mode crystal fiber, and are fabricated usually with a solid
core surrounding by air holes .This properties are very interesting and can have various
application, including high power single mode-fiber lasers and amplifiers.. Since PCFs
deliver more power compare with conventional fiber, they can lengthen the repeater
spacing in telecommunication links. Also since the fiber is made of only silica the fiber
has potential application in sensors and interferometers. Losses associated with this type
of fiber are usually less than 1 dB/km.
Figure 4. Solid core crystal fiber. This type of fiber remains single mode in a wide range of wavelength.
Properties
• Single-mode at all wavelengths • Attenuation as low as < 0.8 dB/km for λ = 1550 nm • Undoped fused silica core and cladding • Near-Gaussian mode profile • Delivery of broadband radiation in a single spatial mode • Short wavelength applications • Sensors and interferometers
Hollow Core Fiber Photonic Bandgap Fibers guide light in a hollow core, surrounded by a
microstructured cladding formed by a periodic arrangement of air holes in silica. Since only a
small fraction of the light propagates in glass, the effect of material nonlinearities is significantly
reduced and the fibers do not suffer from the same limitations on loss as conventional fibers made
from solid material alone. These fibers also have very low bending losses, and very low Fresnel
reflection. Since up to 85 % of the fiber cross section is made of silica, which facilitate the fiber
splicing.
Figure 5. Hollow fiber. This type of fiber has lower loss compare with solid core fiber and is more likely to become e the replacement for conventional long haul optical fiber
Dispersion compensation
A very interesting application of photonic crystal fiber is as dispersion tailoring devices.
For instance, more of the fiber installed were design to operate at 1.3 um, but know
transmission is preffered at 1.55 um and this fiber has considerable dispersion at this
wavelength. Photonic crystal fiber can be employed as dispersion compensation in orde
to eliminate or reduce the dispersion. As a general rule, the bigger the dispersion
compensation, the smaller the PCF length. What distinguish PCF as dispersion
compensation technique compare with other method, is the fact that the range at which
the compensation is made in a PCF (zero dispersion) is broad compare with the other
methods. This is especially important for WDM, where compensation must be done in a
broad range. The range at which compensation can be made is fundamental limited by the
index contrast of the fiber. So, to increase the dispersion the index contrast must be
increased, and this usually done by increasing the air-holes diameter. Figure 6 shows the
dispersion dependences on the air-hole size. This fiber was designed to have zero
dispersion at 1.55 um. The figure embedded illustrates the balances between waveguide
and material dispersion in a PCF. A typical number for the PCF dispersion, is around
2000 ps-nm-1-Km-1, that means that a photonic crystal fiber can compensate 100 times its
length (you only need 1cm of PCF for each meter of regular fiber).
Figure 6. the dispersion slope an be tuned while maintaining a fixed zero dispersion. Insert shows the balance between material and waveguide dispersion for a fiber with zero dispersion slope at 1.55 um.
Polarization maintaining PCF
Birefringence in a optical fiber arises as a consequence of stresses generated in the fiber
during the fabrication process . This anisotropy has a important effect in the transmission
characteristics of the fiber. Birefringence can also be induce by bending and thermal
effect. The net effect is the generation of different optical axes. Plane polarized light
propagating along the fiber will be resolved into components along in theses axes and as
they propagate at different speeds. As result phases differences are created resulting in
elliptically polarized light. Finally, this phases mechanism cause delays in the optical
signal, and this is know as polarization mode dispersion. PMD becomes important at high
transmission rate. Polarization maintaining PCF is emerging as a new competitor for
traditional polarization maintaining fiber. One advantage of PCF is that they remains
single mode and thus can transmit polarized light in a broad range of frequencies. Also
PCF has shorter beat length than common PMF which reduces bend-induced coupling
between polarization states and improve the extinction rate. PCF presents, more stable
temperature coefficient of birefringence. PCF’s find application in sensors gyroscopes
and interferometers. See figure 7.
Figure 7. Polarization maintaining PCF.
The incident field not restrain to the core
Figure 8. Shows the beam propagation method for a solid core, at 1.55 um. Notice that the radius of the incident beam is larger than the core of the fiber. Beam was propagated 100 um.
The incident field not restrain to the core
Figure 9. Shows the beam propagation method for a hollow core, at 1.55 um. Notice that the radius of the incident beam is larger than the core of the fiber. Beam was propagated 100 um
BPM profiles with wavelength dependence
Incident file (constraint to the core)
BPM results a 0.85 nm
Figure 10. Shows the beam propagation method for a hollow core, at 0.85 um. Notice that the radius of the incident beam is larger than the core of the fiber. Beam was propagated 100 um
BPM
Result a 1.3 nm
Result a 1.55 nm
Figure 11. Shows the beam propagation method for a hollow core, at 1.55 um and 1.3 um.
BPM profiles with at 1.55 um at two different number of ring
Incident file (constraint to the core)
Figure 12. Shows the beam propagation method for a hollow core, at 1.55 for two different number of rings
BPM Simulation and Summary In the BPM simulation I am presenting basic result of a propagation of a Gaussian
beam through a Photonic Crystal Fiber. The structure is formed with a cladding
compound of air cylinders with radius of 1.394 um and with a lattice pitch Λ=2.875. I
study two structure, one with hole fiber and the other with a solid core (with a radius of
3.106 um).The first part of the simulation will tend to study the propagation of the beam
in the structure, when the incident beam is not launched into the core but spread through
the structure. It is important to remember that light can not be couple into a slab
waveguide if the radiation is launched into the cladding. As figure 8 and 9 suggest the
light launched in either structure tend to be localized in the core of the fiber after
propagation, which can be advantageous for certain application in which is difficult to
assure that the beam is launched only in the core. The amount of power that remains in
the core for the solid core fiber is around 4.779873e-01 and the coupling efficiency is
0.783340. In the case of the hollow fiber the amount of power within the core is
8.067363e-01 and the coupling efficiency 9.250303e-01. Actually I was expecting to find
better coupling and better confinement for the solid core fiber (since has a higher
effective index in the core of the fiber), that illustrated the complexity of analyze PCF by
standard waveguide concepts.
Second, we want to study what happens when different wavelength are launched into
the structure. By launching light at 1.55, 1.3 and 0.85 um (see figure 11) with field
concentrated in the core of the fiber, we end up with a coupling efficiency of 6.216740e-
01, 6.05419e-01, 6.809596e-01. Notice that the coupling efficiency is higher, if the power
is launched in such a way that is not confined to the core. The reason for that behavior is
not clear but I simply think that by launching the power not confined to the core, the field
is seeing the periodic structure at the very beginning of the simulation, instead when is
confined in the core that have to propagate some distance before seeing any periodic
variation in index of refraction (the size of the core is bigger than the size of the field).
There is another unexpected result, the coupling efficiency is bigger for the
structure with fewer rings (see figure 12). The coupling coefficient for the larger structure
is 6.216740 e-01 and for the smaller is 6.434557e-01. That is hard to understand, since
the bigger the structure the better the response of the fiber. But the answer can be related
with the fact that in the bigger structure there are some defects, since there are some
cylinder missing in certain positions. Those defects can generate phases associated with
them and this can have a important effect on the coupling coefficient. But there is a
important observation we can do; it looks like that increasing the number of rings it does
not affect to much the coupling efficiency but defect can have bigger impact on the
overall coupling efficiency .
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