TECHNIQUES TO COMBAT OSNR IN DWDM LINKS

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2.  DWDM is a technology that combines large number of independent information carrying
wavelengths onto the same fiber and thereby increases the transmission capacity of fiber.
The “spectral bands” where the optical fiber and the transmission equipment can operate
more efficiently are specified by ITU-T as O, E, S, C, L and U bands (from 1260 nm to 1675
nm). While setting up the transmission link, there is a need to ensure that the signal can be
retrieved intelligibly at the receiving end. This can be done preferably by using optical
amplifiers that serve as the key component of a DWDM system. When the signal is amplified
by the optical amplifier (OA), like EDFA, its optical signal to noise ratio (OSNR) is reduced,
and this is the primary reason to have limited number of OAs in a network. One of the
mitigation is to use RAMAN amplifier but it also has some intrinsic noise, though it is less
than that of EDFA.
 The OSNR values that matter the most are at the receiver, because a low OSNR value means
that the receiver will probably not detect or recover the signal. The OSNR limit is one of the
key parameters that determine how far a wavelength can travel prior to regeneration. OSNR
serves as a benchmark indicator for the assessment of performance of optical transmission
systems. DWDM networks need to operate above their OSNR limit to ensure error – free
operation. There exists a direct relationship between OSNR and bit error rate (BER), where
BER is the ultimate value to measure the quality of a transmission. Given the OSNR, the
empirical formula to calculate BER for single fiber is:
Log10 (BER) = 10.7-1.45 (OSNR)
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3. In DWDM links a rule of thumb would be to target an OSNR value greater
than 15 dB to 18 dB at the receiver. OSNR requirements depend on:
Location:
The required OSNR will be different for different locations in the light path. The OSNR
requirement will be higher closer to the transmitter and lower closer to the receiver. This is
because optical amplifiers and reconfigurable add/drop modules (ROADMs) add noise,
which means that the OSNR value degrades after going through each optical amplifier or
ROADM. To ensure that the OSNR value is high enough for proper detection at the receiver,
the number of optical amplifiers and ROADMs needs to be considered when designing a
network.
Type of Network: For a metro network, an OSNR value of >40 dB at the transmitter
might be perfectly acceptable, because there are not many amps between the transmitter and
the receiver. For a submarine network, the OSNR requirements at the transmitter are much
higher.
Data Rate:
With the increase in the data rate for a specific modulation format, the OSNR requirement
also increases.
Target BER:
A lower target BER calls for a higher OSNR value.
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4. The exact requirements at the receiver will vary from one manufacturer to another.
Table 2 displays a few average OSNR figures to guarantee a BER lower than 10-8 at
the receiver [3]:
TABLE 2 Typical OSNR values
Data Rate (Gb/s) 10 40 40 100 100
Modulation
Format
NRZ NRZ DPSK NRZ DPSK
Approx. OSNR
(dB)
11 17 14 21 18
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5. Figure.1 (a) Relation between OSNR, BER and QoS; (b) Impact of poor OSNR
A higher OSNR translates into a lower BER, which equals fewer errors in transmission and
higher quality of service (QoS). The relation and impact of OSNR on system performance
is shown in Fig.1 (a) and (b) .
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6. The optically amplified DWDM networks form the backbone of the long haul and
ultra - long haul commercial terrestrial and submarine systems. This section
discusses the various techniques to enhance the OSNR parameter in DWDM links.
 Use of advanced fiber technology
 Optical Amplifiers
 Cascaded Amplifiers
 Hybrid Amplifiers
 Pumping Methods
 Macro Bending
 Multi-Level Modulation formats
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7.  The OSNR serves as the main constraint in impairment-aware optical routing
and is expressed in general by
Where Pch is the signal power, SSP is the spectral density of ASE noise and BOP bandwidth of optical filter
 Per-channel power Pch is directly proportional to effective area Aeff of the
transmission fiber and the parameter S is directly proportional to the
attenuation coefficient of the fiber. Reducing the loss of the transmission fiber
and components will increase OSNR. Also increasing the launch power per span
is yet another possible approach to enhance OSNR, while maintaining
terrestrial span lengths. However, this can increase the non-linear impairments
such as self-phase modulation (SPM), cross-phase modulation (XPM), four-
wave mixing (FWM) and stimulated Raman scattering (SRS) effects .
 This suggests the approach of enhancing OSNR using advanced fiber
technology wherein fiber with large effective area and ultra-low loss is
fabricated. UltraWaveTM fibers with effective area of 107 µm2 and attenuation
of 0.187dB/km can be considered to improve OSNR for high data rate systems
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8.  Semiconductor optical amplifiers (SOAs) and fiber amplifiers have dominated
their use in the existing 10 Gb/s networks. Semiconductor optical amplifiers
(SOAs) are attractive as they are compact and can be integrated with other
photonic components. However, research reveals that the relatively high
insertion loss and optical signal-to-noise ratio (OSNR) degradation hinder
commercialization of SOAs .
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9.  Another parameter that one should consider in optically amplified transmission system is
the noise factor (NF) of the amplifiers in cascade. By definition
NF (dB) = dB (OSNR)in – dB (OSNR)out
 With NFi and Gi are noise figure and gain in linear units, the total noise factor NFt for a chain of amplifiers can be
written as:
NFt = NF1 + (NF2 – 1)/G1 + (NF2 – 1)/G1*G2 + …..
 The cascading scheme resulting in lower NF i.e. enhances the OSNR.
 For a system containing N fiber spans, where each span is optically amplified, the
simplified OSNR of a 1550 nm signal channel at the end of the system can be expressed as
OSNR [in dB/0.1 nm RBW] = 58 + Pch – Lsp – NF – 10 log10 (N)
where Pch is the per- channel power (in dBm) launched into the span; Lsp is the span loss (in dB).
 The conditions applied to obtain this simplified expression are:
• NF is same for all OAs
• All amplifiers compensate for link loss (Gi = Li)
• All spans have same loss L.
 Increase in OSNR can be achieved by increasing Pch, decreasing NF and decreasing
Lsp. If the OSNR is increased by 3 dB, the length of the system can be doubled,
assuming that the amplifiers are at equal distances and operate in linear region.
Reduction in NF calls for enhancement of OSNR.
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10.  In view of improving the quality of the transferred signal, three typical calculating models
has been proposed of terrestrial DWDM cascaded EDFAs fiber optic communication links
using Hybrid amplifier (HFA) at three locations viz. first, mid and last span. The authors
have selected a combination of Distributed Raman Amplifier (DRA) and EDFA and
proposed general calculating models and suggested algorithm charts to optimize
parameters including signal power per channel launched fiber, EDFAs gain and pump
power of Raman amplifier for improving optical OSNR at the end of link.
 Hybrid amplifiers enhance the transmission capacity of broadband systems, upgrade the
existing systems built with EDFA amplifiers with broader/flatter bandwidth. They
provide an ability to carry more wavelength-multiplexed optical channels at given
spacing among the channels. If Raman amplifiers are chosen for combination with EDFA,
it gives flexibility to the selected band amplification and is less sensitive to nonlinear
effects. Hybrid amplifiers are concerned with maximizing the span length and/or
minimizing the impairments of fiber nonlinearities, enhancing the EDFAs’ bandwidth
and designing “optimal” hybrid amplifiers in order to obtain flat and widest output gain
performance. The gain balance between Raman and EDFAs involves complex problem
with several degrees of freedom (Optimization technique); OSNR, gain-flatness,
bandwidth; number of channels, number of spans and maximum transmission capacity.
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11.  A theoretical investigation about the characterization of RFAs (Raman Fiber
Amplifiers) with bidirectional or co- propagating Raman pump so as to
improve the performance of the amplifier has been proposed . The paper
provides a brief theoretical analysis and does not take into account the taxing
effect of large pump power requirement and also the issues associated with
large pump powers.
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12.  Macro bending effect in optical amplifiers is yet another method to improve the doped fiber
amplifier Gain and Noise Figure [19]. Macro-bending is defined as a smooth bend of fiber
with a bending radius much larger than the fiber radius. Macro-bending modifies the field
distribution in optical fibers and thus changes the spectrum of the wavelength dependent
loss. The macro bending also reduces the noise figure of EDFA at wavelength shorter than
1550. Since keeping the amount of noise low depends on a high population inversion in the
input end of the erbium-doped fiber (EDF), the backward ASE power P –ASE is reduced by
the bending loss. Consecutively, the forward ASE power PASE can be reduced when the
pump power P is large at this part of the EDF which is especially undesirable. This is
attributed and can be described numerically by the following equation:
NF = 1/G + 2PASE/G*hf
where G is the amplifier’s gain, PASE is the ASE power and hf is the photon energy.
There exists an inverse relation between the NF and the OSNR. The noise figure decreases
appreciably due to bending effect.
 From the study it can be concluded that the use of advanced fiber technology to enhance
OSNR faces a limitation posed by the fabrication of fibers with highest possible effective
areas and lowest possible attenuation. For Raman amplification such a system would
demand huge pump powers. Also these fibers may pose incompatibility issues while
splicing them with conventional standard single mode fibers. Whereas macro-bending
technique improves both gain and noise figure by approximately 6 dB and 3 dB,
respectively. The method is cost effective which needs 100mW pump power and does not
require any additional optical components to flatten the gain, thus enables reduction in the
system complexity. www.MapYourTech.com
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13.  Rapid progress has been achieved in transmission systems including multi-level
modulation formats and digital coherent detection techniques to reduce the OSNR
requirement. Especially in conjunction with DPSK modulation formats, use of more
advanced amplification schemes leads to significant improvement in OSNR performance
over conventional EDF only amplification [20].From the literature survey there is further
scope to research in achieving enhancement in OSNR by an optimum configuration of
hybrid amplifiers with the support of the advanced fiber technology.
Data Rate (Gb/s) 10 40 40 100 100
Modulation
Format
NRZ NRZ DPSK NRZ DPSK
Approx. OSNR
(dB)
11 17 14 21 18
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14.  Kazunori Mukasa, Katsunori Imamura, Masanori Takahashi, Takeshi Yagi, Development of novel fibers
for telecoms application, Optical Fiber Technology, Optical Fiber Technology 16, Elsevier-Science
Direct, 2010, 367-377.
 Pierre Sillard, New fibers for ultra-high capacity transport, Optical Fiber Technology (17), Elsevier-
Science Direct, 2011, 495-502.
 Jean-Sébastien Tassé, What should the OSNR values be in DWDM networks? , Optical/Fiber Testing,
Home / Corporate / Blog Home / 2012 / Posted date: 2012-12-06.
 MRV, Application note on DWDM, Optical Communication Systems, 2002.
 Neal S. Bergano, Wavelength Division Multiplexing in Long-Haul Transmission Systems, OFC/NFOEC
2011 SC102.
 Sinclair Vass, Talk on Optical Communications Trends for 2011, JDSU, January 12, 2011. [7] Ted
Schmidt,Christian Malouin,Bo Zhang, RossSaunders,et al, “100G Coherent DWDM
 Transponder Module Enabling Seamless Upgrade of Long Haul Optical Transmission
Systems”,NME2.pdf, OSA/OFC/NFOEC 2010.
 International Journal of Electronics and Communication Engineering & Technology (IJECET),
ISSN 0976– 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 2, February (2014), pp. 10-
20 © IAEME
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