By Neil Lynch
It first discusses the core, cladding and
their refractive indices, the nature of the interface between them (the core
and cladding) which determines their modal properties; we’ll briefly look at
the input signal into the fiber via the proper region of the acceptance angle.
We’ll also look at dispersion as light propagates along the fiber and the
combining effect of material and waveguide dispersions (chromatic dispersion)
on its outcome, followed by orthogonal Polarization, asymmetrical cores,
birefringence (discussed in the video) and 4-wave mixing. We’ll then conclude with
dispersion compensation: special single-mode fibers, chromatic dispersion
compensation (dispersion compensation filters module, tunable dispersion
compensators) and soliton transmission.
“The two key elements of an optical
fiber—from an optical standpoint—are its core
and cladding. The core is the inner
part of the fiber, which guides light. The cladding surrounds it completely, as shown in Figure 1. The refractive
index of the core is higher than that of the cladding, so light in the core
that strikes the boundary with the cladding at a glancing angle (as shown
in figure 2) is confined in the core by total internal reflection” (Hecht,
26). The size of the core and cladding and the nature of the interface between
them determine the fiber’s modal properties and how it transmits light at
different wavelengths. The simple types of fiber have a step-index structure,
where the refractive index changes sharply at the abrupt boundary between a
high-index core and a low index-cladding; of the various transmission modes,
single mode transmission is cleaner and simpler, and it’s also preferred for
fiber-optic systems. The main limitation is that the core[1] of the
fiber must be small enough to restrict transmission to a single mode, yet large
enough to collect most of the input optical signal via the proper region of the
acceptance angle, preferably the half acceptance region above or below the
fiber-axis as shown in figure 3.
“Dispersion is the spreading out of light
pulses as they travel along a fiber. It occurs because the speed of light
through a fiber depends on its wavelength and the propagation mode. The differences
in speed are slight, but like attenuation, they accumulate with distance” (Hecht,
103).
The pulse spreading arises because the
velocity of light through a fiber depends on its wavelength. Material
dispersion causes different wavelengths to travel at different speeds as a
result of variations in the refractive index of the fiber core with wavelength.
Some of the light also travels in the cladding of the fiber, which has a
different refractive index and propagates light at a different speed than the
core; an effect known as waveguide dispersion. Material and waveguide
dispersion combined to produce an overall effect called "chromatic
dispersion" (chromatic dispersion-bb.csis ch.5; web).
If the
circular symmetry of fibers were perfect, polarization would have little
practical impact for communications. However, fiber symmetry is never
absolutely perfect… As a result, the two polarization modes may experience
slightly different conditions and travel along the fiber at slightly different
speeds. This effect is called differential group delay (double-click to see video above, escape to
return (Yao, YouTube)), which averaged
over time becomes polarization-mode dispersion, as shown in figure4. It can cause problems in high performance
systems, such as those transmitting time-division multiplexed signals faster
than about 2.5 Gbit/s.
Standard Single Mode Fiber (SSMF) –
SSMF, also known as non-dispersion shifted fiber, is the most widely used
fiber. SSMF is optimized for minimal dispersion in the 1310 NM band, while the
high dispersion at 1550 NM prevents 4-wave-mixing[3]. With
the assistance of dispersion-compensators[4] (discussed later), long range fiber-transmission distances can be 100 km at
OC-192 bit rates and 525 km at OC-48 bit rates.
The amount of chromatic dispersion
experienced in optical fiber is dependent on the wavelength at which light is
being transmitted, and a graph showing this for regular single-mode fiber is
shown below. It is worth noting that there is a "slope" to the
dispersion; meaning that each wavelength experiences a different amount of
dispersion.
Normally this change is very small over the range of wavelengths generated by a single laser transmitter. However, it’s important in wavelength-division multiplex systems, which carry many optical channels spanning tens of nanometers in wavelength.
Specification
sheets typically do not plot chromatic dispersion directly as a function of
wavelength, but give the chromatic dispersion that may be found at a range of
wavelengths (Hecht, 110).
Special Single-Mode Fibers
Special
single-mode fibers can control the polarization of light they transmit. There
are two types: true single-polarization fiber and polarization-maintaining
fiber (see video below). Both intentionally avoid circular symmetry, so they
transmit vertically and horizontally polarized light differently…
Single-polarization fibers can remove
the undesired polarization in order to achieve, a transmission … almost as well
as standard single-mode fiber … where only the desired polarization remains in
the end.
Under the proper conditions, a single-polarization fiber attenuates the undesired polarization by a factor of 1000 to 10,000 within a few meters but transmit the desired polarization almost as well as standard single-mode fiber.
Polarization-Maintaining Fiber (Yao, YouTube) double-click for video, escape to exit
|
… Polarization entanglement could be
significantly decohered (degraded) during fiber transmission due to two
polarization effects in optical fibers: polarization mode dispersion (PMD) and
polarization-dependent loss (PDL); fiber symmetry is never absolutely perfect…
as a result, these polarizations may experience slightly different conditions
and travel along the fiber at slightly different speeds.
By introducing PMD in a controlled
way and performing tomography for various levels of PMD in each fiber,… PMD induced
degradations:
Loss of polarization
entanglement in a fiber-optic system with polarization mode dispersion in one
optical path, and either reduce or increase decoherence depending on the
relative orientation of two PMD elements. Sometimes in the latter case the
entanglement disappears completely, which is a manifestation of the sudden
death arising naturally during photon propagation in fibers (Hecht, 86).
… Pulse dispersion is cumulative, building up along the
length of a fiber system. In general, this means that adding more fiber only
makes pulse dispersion worse. However, it is possible to reduce total chromatic
dispersion by adding a length of fiber with chromatic dispersion of the
opposite sign. For example, you could add a length of fiber with negative
chromatic dispersion at 1550 nm to a system containing fiber with positive
dispersion in that band. According to Downing, “… if the existing fiber dispersion parameter is
-8 ps, then the total dispersion can be brought to zero by adding a length with
net dispersion of +8 ps” (281). This idea is similar to using waveguide
dispersion to offset material dispersion, but in this case the compensation is
done by splicing together two fibers with different chromatic dispersion[5].
The dispersion-compensation fiber could
be added in a length of cable, but it’s often installed in modular form in an
equipment rack near a receiver or optical amplifier. In long-distance systems,
length of the two types of fibers alternate, so chromatic dispersion does not
build up to excessive levels before being reduced.
Chromatic Dispersion Compensation
The
demand for DCMs [(chromatic) Dispersion Compensation Modules] will quickly grow
in the near future, driven by the development of ultra-long-haul and 40 Gbps systems.
Both applications require greater numbers of DCMs per link and higher priced,
higher performance devices having 100 percent slope compensation, and in some
cases, dynamically tunable slope compensation (see tunable dispersion
compensator below).
| n Modul |
In optical fibers, waveforms
lengthen over long distances, making these signals difficult to interpret when
they reach the receiving end. The result is distorted data signals that
represent transmission errors at the intended receiver.
As network speeds and span lengths
increase; (data rates and coverage areas of telecom systems) … new technical
challenges are appearing, including the adverse effects of signal broadening
caused by chromatic dispersion. To counteract theses phenomenon’s the
importance of tunable dispersion compensation should not be underestimated,
especially in solving chromatic dispersion in fast growing OC 192 networks and
the developing area of polarization mode dispersion. Tunability is critical in
allowing networks of the future to adapt to variable path factors,
environmental changes, and configurations that are themselves in constant
change.
Multichannel capability - In order to realize
the potential of OC 192 and OC 768 DWDM networks, DCM solutions featuring
multichannel capability are vital. Because OC 192 and OC 768 can have up to
several dozen channels, a decreasing amount of space on telecommunications
racks requires multichannel capability in dispersion compensation solutions.
In the search for dispersion
compensation solutions, DCMs are a great potential in solving the challenges
associated with multichannel, high-speed networks. Recent innovations with
components such as fiber-Bragg-gratings[6], coupled
with important developments in tunability and multichannel capabilities, are
important factors for the industry's future.
Dispersion Compensating Filter Modules are passive optical
fiber devices used in various locations within the optical fiber communications
network link.
Telecommunication System operators are upgrading their links
toward 10 Gbps transmission speeds and the use of dense wavelength division
multiplexing (DWDM), to increase their network capacity. Often, chromatic
dispersion develops into a critical variable in expanding the capacity, since
existing (installed) standard optical fiber with varying quality, is optimized
for single wavelength transmission at 1310 NM
Dispersion compensating filter modules will be used as an
alternative to installing a specific length of dispersion compensating optical
fiber (DCF) in the link ( DCFs are usually installed in small spools).
Dispersion compensating filter modules will also be used in
conjunction with Optical Add/Drop Multiplexers (OADM). The OADM segments
wavelengths at specified locations in the network link, and these wavelengths
require dispersion compensation before reaching the receiver (Dispersion
Compensation Modules - bb.csis chapter 5, web).
Solition Transmission
According to (blackboard csis document), there is a relatively new fiber optic data transmission scheme that utilizes something called Soliton Pulses. These are very short bursts of light generated in an Erbium-doped Fiber LASER. Soliton light can be used to transmit data at rates in excess of 50 Gb/s, at distances over 19,000 km of Dispersion-Shifted Fiber, requiring no repeaters, and with no errors. This data rate is the equivalent of sending 6,200 bibles per second. At this rate, one bible could be sent to everyone on earth--6 billion people--in about 10 days.
·
The soliton is a wave that exists in nature
which can propagate over long distances without any distortion of its waveform.
Optical solitons in optical fiber are maintained by the balance of the
nonlinear optical index and the group velocity dispersion of the fiber. Ideal
solitons, which propagate without waveform distortion, can exist only in a
transmission line with no energy loss and no fluctuation of the grou
Soliton pulses are very short pulses of
Hecht, Jeff.
Understanding Fiber Optics 5/e. New
Jersey: Pearson Education, Inc., 2006. Print. (Light Guiding, p.26)
Fiber Optic
Cables. Illustration: Figure 1 and 2. Retrieved from http://www.legrand.us/cablofil/tech_resources/fiber-optic-cable.aspx
Hecht,
Jeff. Understanding Fiber Optics 5/e. New Jersey: Pearson Education, Inc., 2006. Print. (Dispersion, p.103)
Chromatic
Dispersions. Blackboard Course-Documents Chapter 5. Retrieved from http://bb.csis.pace.edu/course/1/AIT371127a02/content/_162494_1/dir_chapter05.zip/ch5.htm
Yao, Colin. Polarization
Mode Dispersion. YouTube. Retrieved from. http://www.youtube.com/watch?v=J4- wCa_VNfA&feature=player_embedded#
Hecht,
Jeff. Understanding Fiber Optics 5/e. New Jersey: Pearson Education, Inc., 2006. Print. (Polarization Mode
Dispersion, p.113)
Positive
and Negative Dispersions. Blackboard Course-Documents Chapter 5. Retrieved
from http://bb.csis.pace.edu/course/1/AIT371127a02/content/_162494_1/dir_chapter05.zip/ch5.htm
Hecht,
Jeff. Understanding Fiber Optics 5/e. New Jersey: Pearson Education, Inc., 2006. Print. (4-Wave Mixing, p.117)
Hecht,
Jeff. Understanding Fiber Optics 5/e. New Jersey: Pearson Education, Inc., 2006. Print. (Dispersion Slope, p.110)
Orthogonal
Polarization. Blackboard Course-Documents Chapter 4. Retrieved from http://bb.csis.pace.edu/course/1/AIT371127a02/content/_162486_1/dir_chapter04.zip/ch4.htm
Yao, Colin. Polarization
Maintaining Fiber. YouTube. Retrieve from. http://www.youtube.com/watch?v=7rrb-_Iin-g
Hecht,
Jeff. Understanding Fiber Optics 5/e. New Jersey: Pearson Education, Inc., 2006. Print. (Polarization Mode
Dispersion, p.86)
Downing,
James. Fiber Optic Communications:
Delmar/Cengage, 2005. Print. (Dispersion Compensation, 281)
Dispersion
Compensation Modules. Blackboard Course-Documents Chapter 5. Retrieved
from http://bb.csis.pace.edu/course/1/AIT371127a02/content/_162494_1/dir_chapter05.zip/ch5.htm
Soliton Transmission. Blackboard Chapter 19 Document. Retrieved
from http://bb.csis.pace.edu/courses/1/AIT371127a02/content/_162560_1/ch19overview_1.htm
[1]The most common dimensions of optical fibers are 9/125, 50/125, 62.5/125 and 100/140 microns.
[2] A chirp is a signal in which the frequency increases
('up-chirp') or decreases ('down-chirp') with time. In some sources, the term
chirp is used interchangeably with sweep signal … In optics, ultra short laser
pulses also exhibit chirp due to the dispersion of the materials they propagate
through.
[3]
4-Wave
mixing: multiple optical channels passing through the same fiber interact with
each other only very weakly, making wavelength-division multiplexing possible.
However, these weak interactions in glass can become significant over long
fiber-transmission distances. The most important is four wave mixing (sometimes
called four-photon mixing) in which three wavelengths interact to generate a
fourth (Hecht, 117).
[4] Dispersion
compensation: as data rates of telecom systems increase from 10 to 40 Gbit/s,
new technical challenges appear, including the adverse effects of signal
broadening caused by chromatic dispersion. This physical phenomenon originates
from the wavelength dependence of the propagation velocity in the transport
optical fiber. In such a material, the blue part of an optical pulse propagates
faster than its red part, resulting in progressive pulse broadening.
Fortunately, it is easy to recompress the optical
pulses by providing a device that does just the opposite: providing a longer
propagation time for the blue part than for the red part of the optical pulses.
This scheme is referred to as dispersion compensation and actually works better
than trying to eliminate the chromatic dispersion in the transport fiber. For
example, dispersion-shifted fiber (DSF) was developed to provide negligible
chromatic dispersion, but brings new problems, including larger nonlinear
effects (Avants, web).
[5] Chromatic dispersion is measured in Ps/NM/km. This
means for every km of fiber traveled through, a pulse with a 1 NM spread of
wavelengths will disperse by 1 Ps (Ps = picosecond = 1 x 10^-12 second) for a
dispersion of 1 Ps/NM/km. Therefore you can see that with a 1 Ps/NM/km
chromatic dispersion, a 10-Gbit/s pulse with a 0.2nm spectral width will have
spread by a whole bit period (100 Ps) after 500 km of fiber and will then be completely
indistinguishable (chromatic dispersion, bb.csis chapter 5; web).
Proposed for dispersion compensation two decades ago, a
fiber Bragg grating (FBG) consists of a longitudinal index modulation in the
core of an optical fiber.1 The light is reflected by the FBG when its
wavelength satisfies the interference condition dictated by the modulation
period. For dispersion-compensation purposes, the modulation period varies
along the fiber axis such that the blue part and red part of an optical pulse
are reflected back at the far and front parts of the FBG, respectively.
Since the FBG-compensation idea was first conceived-and
especially over the past five years-significant advances have been made. This
technology is now mature enough for dispersion compensation and represents the
first deployed compensation technology besides DCF. The major advantage of FBG
technology is its ability to provide tunable dispersion compensation-a critical
feature required for 40 Gbit/s communication systems that cannot be met by DCF.
Avants: Laserfocusworld. Retrieved from http://www.laserfocusworld.com/articles/print/volume-43/issue-1/features/dispersion-compensation-fbgs-enhance-dispersion-compensation.html
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