Key to understanding the effect of chromatic dispersion is the characteristics
of optical sources. DFB (distributed feedback) lasers have a built-in grating
that limits output to discrete lines 0.05 nm wide. The figure of merit that
describes their wavelength purity is the relative intensity of side mode lines
rather than peak width. In VCL and Fabry-Perot lasers without a frequency
selective element in the laser cavity, the modes are much more closely spaced
and essentially form a continuum as is shown below. The DFB design can also
be used in 1310 nm emitters at some cost penalty.
|
|
Figure 2:
Laser output power and spectral– width comparisons.
|
|
The figure is taken from http://www.commsdesign.com/main/2000/02/0002feat2.htm
by Scott Kip.
|
From "Optical Network Design and Implementation" by
Vivek Alwayn. This is a part of the free sample chapter.
Fiber Types
This section discusses various MMF and SMF types currently used for
premise, metro, aerial, submarine, and long-haul applications. The
International Telecommunication Union (ITU-T), which is a global
standardization body for telecommunication systems and vendors, has
standardized various fiber types. These include the 50/125-u graded
index fiber (G.651), Nondispersion-shifted fiber (G.652), dispersion-
shifted fiber (G.653), 1550-nm loss-minimized fiber (G.654), and
NZDSF (G.655).
Multimode Fiber with a 50-Micron Core (ITU-T G.651)
The ITU-T G.651 is an MMF with a 50 micron nominal core diameter and a
125-u nominal cladding diameter with a graded refractive index. The
attenuation parameter for G.651 fiber is typically 0.8 dB/km at 1310 nm.
The main application for ITU-T G.651 fiber is for short-reach optical
transmission systems. This fiber is optimized for use in the 1300-nm
band. It can also operate in the 850-nm band.
Nondispersion-Shifted Fiber (ITU-T G.652)
Production volumes starting in 1984
Improved 1550 nM attenuation (0.25 db/km) in 1989
Even better (0.20 db/km) in 1992
Controlled low PMD since 1996
PMD = Polarization mode dispersion, caused by eliptical instead of
perfectly round fiber.
The ITU-T G.652 fiber is also known as standard SMF (Single Mode Fiber)
and is the most commonly deployed fiber. This fiber has a simple
step-index structure and is optimized for operation in the 1310-nm band.
It has a zero-dispersion wavelength at 1310 nm and can also operate
in the 1550-nm band, but it is not optimized for this region. The
typical chromatic dispersion at 1550 nm is high at 17 ps/nm-km.
Dispersion compensation must be employed for high-bit-rate
applications. The attenuation parameter for G.652 fiber is typically
0.2 dB/km at 1550 nm, and the PMD parameter is less than 0.1 ps/km.
An example of this type of fiber is Corning SMF-28.
note: This is the fiber type for the 802.3z Gig-E LX fiber specification.
Cisco specifies G.652 for 10 Gig-E modules operating at 1550 nm. A footnote
claims that for unamplified links, range is controlled by attenuation and
the power limit rather than dispersion.
Low Water Peak Nondispersion-Shifted Fiber (ITU-T G.652.C)
circa 2000
The legacy ITU-T G.652 standard SMFs are not optimized for WDM
applications due to the high attenuation around the water peak
region. ITU G.652.C-compliant fibers offer extremely low attenuation
around the OH peaks. The G.652.C fiber is optimized for networks
where transmission occurs across a broad range of wavelengths
from 1285 nm to 1625 nm. Although G.652.C-compliant fibers offer
excellent capabilities for shorter, unamplified metro and access
networks, they do not fully address the needs for 1550-nm
transmission. The attenuation parameter for G.652 fiber is
typically 0.2 dB/km at 1550 nm, and the PMD parameter is
less than 0.1 ps/ km. An example of this type of fiber is Corning
SMF-28e.
Low Water Peak Nondispersion-Shifted Fiber (ITU-T G.652.D)
March 2003
There are four tables in the standard. A and B have a water peak.
C and D eliminate the water peak for full spectrum operation.
D has controlled PMD.
FIA -- Fiberoptic Industry Association
OS1 Optical single mode type 1
This is cable made with G.652 fiber.
Maximum attenuation at 1300 or 1500 nM is 1 dB/Km.
OS2 Optical single mode type 2
This is cable made with G.652C fiber.
Spec'd at 0.4 db/Km max, it must be loose tube construction to
achieve the good attenuation. Also low water, so useful for
metro CWDM applications over a broad wavelength range.
OM1 Optical multimode type 1
This is FDDI grade 62 micron core fiber. Intended for use at
1300 nM, it has modal bandwidth of 500 MHz-Km. Performance
at 850 nM is degraded: 200 MHz-Km.
OM2 Optical multimode type 2
50 micron step index fiber with improved modal bandwidth at
850 nM of 500 MHz-Km.
OM3 Optical multimode type 3
Laser enhanced 50 micron fiber is characterized for use with
a collimated launch that does not fill the fiber aperature.
Overfilled launch BW is 1500 MHz-Km which improves to 2000
MHz-Km with a VCEL laser at 850 nM. OM3 cable jacket is usually
color-coded aqua or turquoise.
Dispersion-Shifter Fiber (ITU-T G.653)
Conventional SMF has a zero-dispersion wavelength that falls near
the 1310-nm window band. SMF shows high dispersion values over
the range between 1500 nm and 1600 nm (third window band). The
trend of shifting the operating transmission wavelength from 1310
nm to 1550 nm initiated the development of a fiber type called
dispersion-shifted fiber (DSF). DSF exhibits a zero-dispersion
value around the 1550-nm wavelength where the attenuation is
minimum. The DSFs are optimized for operating in the region
between 1500 to 1600 nm. With the introduction of WDM systems,
however, channels allocated near 1550 nm in DSF are seriously
affected by noise induced as a result of nonlinear effects caused
by FWM. This initiated the development of NZDSF. Figure 3-14
illustrates the dispersion slope of DSF with respect to SMF and
NZDSF. G.653 fiber is rarely deployed any more and has been
superseded by G.655.
Figure 3-14 Fiber Dispersion Slopes
1550-nm Loss-Minimized Fiber (ITU-T G.654)
The ITU-T G.654 fiber is optimized for operation in the 1500-nm
to 1600-nm region. This fiber has a low loss in the 1550-nm band.
Low loss is achieved by using a pure silica core. ITU-T G.654
fibers can handle higher power levels and have a larger core
area. These fibers have a high chromatic dispersion at 1550 nm.
The ITU G.654 fiber has been designed for extended long-haul
undersea applications.
Nonzero Dispersion Shifted Fiber (ITU-T G.655)
Prestandard versions introduced in 1996.
Using nonzero dispersion-shifted fiber (NZDSF) can mitigate
nonlinear characteristics. NZDSF fiber overcomes these effects by
moving the zero-dispersion wavelength outside the 1550-nm operating
window. The practical effect of this is to have a small but
finite amount of chromatic dispersion at 1550 nm, which minimizes
nonlinear effects, such as FWM, SPM, and XPM, which are seen in
the dense wavelength-division multiplexed (DWDM) systems without
the need for costly dispersion compensation. There are two fiber
families called nonzero dispersion (NZD+ and NZD), in which the
zero-dispersion value falls before and after the 1550-nm
wavelength, respectively. The typical chromatic dispersion for
G.655 fiber at 1550 nm is 4.5 ps/nm-km. The attenuation parameter
for G.655 fiber is typically 0.2 dB/km at 1550 nm, and the PMD
parameter is less than 0.1 ps/ km. The Corning LEAF fiber is an
example of an enhanced G.655 fiber with a 32 percent larger
effective area. Figure 3-14 illustrates the dispersion slope of
NZDSF with respect to SMF and DSF.
From the ITU - One more type
30 April 2004, Geneva - ITU has set a global standard for a new
optical fibre that will make it easier for network operators to
deploy bandwidth to maximise technology in core networks. The
development of standards in this area is important if network
operators are to reduce costs and provide more innovative
services to customers.
"G.656 is another significant step in the evolution of optical
networks, because it allows a more economical deployment of
optical transport networks", says Peter Wery, Chairman of ITU-T
Study Group 15 responsible for the Recommendation. The new standard
Recommendation G.656 will allow the easier deployment of Coarse
Wave Division Multiplexing (CWDM) in metropolitan areas, and
increase the capacity of fibre in Dense Wave Division Multiplexing
(DWDM) systems. Wave Division Multiplexing (WDM)increases the
data carrying capacity of an optical fibre by allowing simultaneous
operation at more than one wavelength.
G.656 allows operators using CWDM to deploy systems without the
need to compensate for chromatic dispersion, a phenomenon that at
low levels counteracts distortion, but at high-levels can make a
signal unusable. Although complicated, the management of chromatic
dispersion is crucial as the number of wavelengths used in WDM
systems increase. ITU has a history of providing the specifications
that allow operators to most efficiently handle this.
G.656 also means that at least 40 more channels can be added to
DWDM systems. In this case chromatic dispersion is used to
control harmful interference over this unprecedented range of
the optical spectrum.
Note to technical editors:
The most important new feature in Recommendation G.656 fibre is
the chromatic dispersion coefficient. In G.656 this coefficient
has an allowed range of 2 to 14 ps/nm*km in the 1460-1625 nm
band, compared to 1 to 10 ps/nm*km for G.655.B and G.655.C which
is only related to the 1530-1565 nm band. This low value of the
chromatic dispersion coefficient in the S-C-L bands is the real
novelty of G.656 because it allows the utilization of a larger
wavelength band.
The other characteristics are very similar to previous Recommen-
dations. The range of mode field diameter permitted in G.656 of 7
to 11 5m compares to 8 to 11 5m in the G.655 non-zero
dispersion-shifted fibre. G.656 fibre has a maximum PMD link
design value of 0.20 ps/sqrtkm, which is the lowest value recom-
mended by ITU-T (the same value that ITU-T recently adopted for
G.655.C). G.656 has the same cable cut-off wavelength and cable
attenuation coefficients in the C and L bands as G.655.
ITU-T G.656 (Characteristics of a fibre and cable with Non-Zero
Dispersion for Wideband Optical Transport) is the most recent in
the G-series which specifies the geometrical, physical, mechani-
cal and transmission characteristics of the optical fibres. Other
Recommendations in this series include:
Yet another ITU addition G.657
December 2006 -- Cooked up for the cable TV and FTTH industries.
It is hard to control bend radius in the field. This fiber is
designed for installation abuse.
G.657 Cat A -- Max 0.75 dB for a full turn around a 2 cm mandrel
G.657 Cat B -- Max 0.5 dB for a full turn around a 1.5 cm mandrel
Both at 1550 nm
ITU-T G.652 - Characteristics of a single-mode optical fibre and
cable
ITU-T G.653 - Characteristics of a dispersion-shifted single-mode
optical fibre and cable
ITU-T G.654 - Characteristics of a cut-off shifted single-mode
optical fibre cable
ITU-T G.655 - Characteristics of a non-zero dispersion-shifted
single-mode optical fibre and cable
There's more to fiber than meets the eye
This text is a copy of a July 2002 artice in Fibers.org. Article
by Richard Ednay, some notes by Erik Radius
17 July 2002
Richard Ednay, technical director of Optical Technology Training
(OTT) in the UK, is one of Europe's foremost authorities on
fiber-optics training for technicians, engineers and senior
management from all sectors of the optical comms industry. In his
July 2002 column for fibers.org, he reviews the different types
of optical fiber found in telecom networks in an effort to bring
some much-needed clarity to an often confusing state of affairs.
There seems to be a lot of misunderstanding of new fiber types
across the industry - even some confusion about what to call
them. For example, G.652, SMF-28, CW1505x, NDSF - these terms
could all apply to the same fiber. G.652 is an International
Telecommunication Union (ITU-T) recommendation number; SMF-28 is
a manufacturer's (Corning) brand name; CW1505x is an example of a
customer's (BT) specification; and non-dispersion-shifted fiber
is a description of the product's distinguishing technical
characteristics. No wonder people get confused - and that's just
talking about the "standard" singlemode fiber that's been around
for years.
Generally, our training courses here at OTT concentrate on offi-
cial standards - so we tend to refer to the different fiber types
by their ITU-T designations. What follows is a brief rundown of
the various fiber types and what they're best at. Bear in mind,
though, that the fiber standards highlighted here have been put
together by committees, whose conclusions generally represent the
lowest common denominator in terms of fiber performance. In most
respects, leading manufacturers should be able to beat the
minimum performance requirements with ease.
G.652: This is the original singlemode fiber with a simple
step-index structure. It has zero chromatic dispersion near 1310 nm
and works very well at that wavelength. While this is fine for
applications over moderate distances (up to 50 km), the fiber's
lowest-loss wavelengths are around 1550 nm for long-reach systems
-- which complicates things somewhat. Incidentally, the current
version of the ITU recommendation has three different grades of
performance specified for different applications.
G.653: Dispersion-shifted fiber was developed to address this
conflict between best bandwidth at one wavelength and lowest loss
at another. So, using a more complex structure in the core region
and a very small core area, the wavelength of zero chromatic
dispersion was shifted up to 1550 nm to coincide with the lowest
losses in the fiber. The result? High-bandwidth, long-distance
transmission operating in the 1550 nm window. But there's a
catch: G.653 only works well for single-channel systems.
Unfortunately, the small-core characteristics of G.653 aren't
really compatible with two technologies which, over the course of
the past decade, have become key building blocks of the fiber-
optic network - i.e. the optical-fiber amplifier and dense
wavelength-division multiplexing (DWDM). Fiber amplifiers give us
much higher power levels, allowing extended transmission distances.
DWDM, on the other hand, provides much higher bandwidth by
squeezing a large number of wavelength channels down the same
fiber simultaneously.
The G.653 problem arises as a result of a high power concentra-
tion in the fiber core, which in turn generates nonlinear ef-
fects. One of the most troublesome, four-wave mixing, occurs in a
DWDM system with zero chromatic dispersion, causing unacceptable
crosstalk and interference between channels.
G.655: This was developed as a fiber type that's optimized for
long-haul DWDM transmission at wavelengths of around 1550 nm. It
has a small, controlled amount of chromatic dispersion in the C-
band (1530-1560 nm), where amplifiers work best, and has a larger
core area than G.653 fiber. These characteristics combat the
problems associated with four-wave mixing and other nonlinear ef-
fects. This fiber type is known as non-zero dispersion-shifted
fiber (NZDSF).
G.654: "What about G.654? You've missed that one out!" is a ques-
tion that regularly crops up at this point in our training
courses. G.654 is a specialty fiber that was developed primarily
for subsea applications. It can handle higher power levels, hav-
ing a larger core area, but usually also has high chromatic
dispersion at 1550 nm. It is not designed to operate at 1310nm at
all.
G.651: For completeness, we probably ought to mention G.651. This
is a multimode fiber with a 50 micron core. It's not currently
found much in telecom systems, though it could make a comeback in
access networks [note: Bredband used this fiber type in a Fiber-
to-the-Home pilot network in Eindhoven. I designed it ;-) ERa].
Clearly, in a short column like this, it's impossible to deal ex-
haustively with all of the ramifications of the different fiber
types, let alone the variations that exist between different ven-
dors' offerings for each class of fiber.
The bottom line is this: there is no substitute for information.
In other words, always make sure that you know which fiber type
you're dealing with and treat it accordingly.
Cisco's wavelength graph
| First window | 800 - 900 nm |
| Second window | 1250 - 1350 nm |
| C-band | 1530 - 1565 nm |
| L-band | 1570 - 1610 nm |
|
This table lifted from a Cisco data sheet https://www.cisco.com/c/en/us/products/collateral/interfaces-modules/transceiver-modules/data_sheet_c78-660083.html
It highlights that you can get four lanes either with four fiber strands, or
with wavelength multiplexing.
|
| Product | Type | Connector Type |
| QSFP-40G-SR4-S | 40GBASE-SR4, 4 lanes, 850 nm MMF | MPO-12 |
| QSFP-40G-LR4-S | 40GBASE-LR4, 1310 nm, SMF | LC |
| QSFP-40G-SR-BD | 40GBASE-SR-BiDi, duplex MMF | LC |
| QSFP-40G-BD-RX | 40GBASE-SR-BiDi, duplex MMF, Monitor | LC |
| QSFP-40G-SR4 | 40GBASE-SR4, 4 lanes, 850 nm MMF | MPO-12 |
| FET-40G | Fabric Extender, 4 lanes, 850 nm MMF | MPO-12 |
| QSFP-40G-CSR4 | 40GBASE-CSR4, 4 lanes, 850 nm MMF | MPO-12 |
| WSP-Q40GLR4L | 40GBASE-LR4-Lite, 1310 nm, SMF | LC |
| QSFP-4X10G-LR-S | 4x10GBASE-LR | MPO-12 |
| QSFP-40G-LR4 | 40GBASE-LR4, 1310 nm, SMF with OTU3 data-rate support | LC |
| QSFP-40G-ER4 | 40GBASE-ER4, 1310 nm, SMF with OTU3 data-rate support | LC |
| QSFP-4SFP10G-CU (0.5M,1M,2M, 3M,4M, 5M) | QSFP to 4 SFP+ copper break-out cables | - |
| QSFP-4X10G-AC (7M, 10M) | |
| QSFP-H40G-CU (0.5M,1M,2M, 3M,4M, 5M) | QSFP to QSFP copper direct-attach cables | |
| QSFP-H40G-ACU (7M, 10M) | |
| QSFP-4X10G-AOC (1M, 2M, 3M, 5M, 7M, 10M) | QSFP to four SFP+ active optical breakout cables | |
| QSFP-H40G-AOC (1M, 2M, 3M, 5M, 7M, 10M, 15M, 20M, 25M, 30M) | QSFP to QSFP active optical cables | |
| CVR-QSFP-SFP10G | QSFP 40G to SFP+ 10G adapter | |