TECHNIQUES TO COMBAT OSNR IN DWDM LINKS

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COMBATING OSNR IN DWDM LINKS!

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INTRODUCTION

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)


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.


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


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) .


TECHNIQUES TO IMPROVE OSNR IN DWDM LINKS

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


Use of advanced fiber technology 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


Optical Amplifiers

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 .


Cascaded Amplifiers Another parameter that one should consider in optically amplified transmission system is

the noise factor (NF) of the amplifiers in cascade. By definitionNF (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.


Hybrid Amplifiers 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.


Pumping Methods

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.


Macro Bending 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*hfwhere 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.


Multi-Level Modulation formats

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


References 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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TECHNIQUES TO COMBAT OSNR IN DWDM LINKS

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