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Power Booster EDFA Extending Single-span Network Reach

日期: 2017-02-20
浏览次数: 64

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1 Introduction

Geography often limits the ability to provision repeater sites between two remote terminal locations. For example, submarine links (island hopping, oil rigs), links spanning over large unpopulated areas (deserts, jungles, mountain ranges), and single-span disaster recovery solutions for enterprise storage systems. Even when it is not imperative, a single-hop solution is often desired as it reduces operational expenses and increases security.


The optical networking industry is aligning itself to this growing need by searching for practical, realizable and cost-effective solutions. These include: advanced modulation techniques, improved FEC algorithms, or advanced optical amplification techniques.


In this paper we focus on advanced optical amplification techniques, and show how they provide a costeffective method for increasing single-span system reach without requiring significant system redesign.


2 Technology Alternatives for Increasing the Span Limit

Several technology alternatives exist to increase the span limit. Advanced modulation techniques are a common method for Ultra Long Haul (ULH) equipment manufacturers to increase fiber link length by improving the tolerance to Optical Signal to Noise Ratio (OSNR) and non-linear effects. The Non-Return to Zero (NRZ) data coding is still the most common and is a widely used coding technique in Metro to Regional optical networking applications. NRZ is the lowest cost technique, but in terms of OSNR and non-linear tolerance, it is inferior to options such as Return-to-Zero (RZ) or RZ - Differential Phase-Shift Keying (RZDPSK). However, these advanced modulation schemes are either still confined to the laboratory or implemented only in high-end ULH systems. Introducing even just RZ to an existing system will require a significant increase in capital expense.


Another method to realize greater transmission distances is Forward Error Correction codes (FEC). FEC significantly increases the system margin by embedding extra data that is used to correct errors, at the expense of a slight increase in signal bandwidth. The most common FEC algorithm is G.709 Reed-Solomon (RS-FEC). Reach can be further increased by implementing other FEC algorithms such as enhanced FEC, super FEC, or breakthrough algorithms such as soft decision turbo product code FEC1 . FEC is an excellent low-cost method for increasing span lengths and is almost mandatory in today's competitive high-end optical networks.


Advanced optical amplification techniques can also improve span power budget, without significantly modifying existing terminal equipment. Amplification solutions may include simply increasing the available output power of the standard booster, adding Distributed Raman Amplification (DRA) or using Remote Optically-Pumped Amplifiers (ROPA). Optical amplifier solutions often require increased day-one CapEx, but on a cost per-channel basis they are a good cost effective alternative. Increasing the standard booster power may be easily achieved by using Finisar's UltraSpan Power Booster EDFA - a special low gain, high output power amplifier placed after any existing booster. This stand-alone Power Booster is an excellent alternative for increasing system margin without major modification to existing equipment. Reach can be further improved with the addition of a stand-alone DRA. ROPA can also significantly increase the span power budget, but may not always be an option for pre-deployed fiber links which commonly do not have adequate flexibility to optimize the EDF placement.


Since extending a single-span budget is an occasional but sometimes unavoidable requirement, it is advantageous for the system vendor to be able do this with existing equipment without system redesign and unnecessary additional capital expense to the terminal equipment. For today's systems, this can be easily achieved using a stand-alone Power Booster EDFA, DRA, and ROPA. In this article we will explore the design considerations of adding these modules to an existing system.


3 Increasing Span Limit Using Optical Amplifiers

To increase the span limit using optical amplifiers, four configurations will be discussed. First, we will explore the limitation of existing standard booster and pre-amp configuration, then we will see how adding a Power Booster EDFA can help in multi-channel systems, and finally, how a DRA and ROPA can further improve the link budget.


3.1 Limited Booster Power

Typically, the limiting factor in long links is OSNR. When the signal power received at the end of the link is reduced due to high span attenuation, the OSNR is also reduced. However, every transmission system has its minimum OSNR tolerance. Typical 2.5Gb/s and 10Gb/s NRZ transmission systems without FEC will have an OSNR tolerance of about 16dB and 21dB, respectively. When FEC is implemented, the bit-rate is increased to 2.7Gb/s and 10.7Gb/s, but the resulting OSNR tolerance will reduce to approximately 10dB and 13dB, respectively. These figures are practical boundaries (not including margins or penalties due to effects such as dispersion) of OSNR tolerances in the majority of deployed systems.


For a single-span link containing a pre-amplifier and booster, as shown in Figure 1, the noise is mainly generated within the pre-amplifier.


In this configuration the OSNR at the end of the link can be increased by increasing the booster output power - every dB increase in booster power results in an immediate dB improvement in OSNR. However, this can be done only to a limit, as non-linear effects will start deteriorating the signal when the launch power increases beyond a certain point. For a single-span, single-channel applications, a standard booster with output power of up to 20dBm is usually sufficient for maximizing the output power and hence the OSNR, but vendors should be aware of the non-linear limits of their system. The influence of non-linear effects will be discussed shortly.


3.2 Span Extension Using a Power Booster EDFA

When the system includes more than one channel and the total output power is limited by the booster output power, the overall per-channel launch power is reduced by 10logN, where N is the number of channels. Therefore, to further increase the per-channel launch power to maximize link budget, a Power Booster EDFA can be added, as shown in Figure 2. This innovative amplifier can provide a desirable solution for many single-span applications, when designed from standard, mature and telecom-qualified single-clad EDF technology.


A standard booster has output power of 17dBm-20dBm, a Power Booster can be used to boost the total output power to 26dB, thereby increasing the per-channel signal power. However, for these power levels, non-linear effects begin to deteriorate the signal. Therefore, it is important to determine the maximum launch power limit of the signal.


For a single-span transmission link containing few, broadly-spaced channels (up to 8), with 2.7Gb/s and 10.7Gb/s data rates, two non-linear effects are relevant: Stimulated Brillouin Scattering (SBS) and SelfPhase Modulation (SPM). The Four Wave Mixing (FWM) effect is usually negligible, and can be eliminated by avoiding low dispersive fiber or by distributing the signal wavelengths unevenly.


When increasing signal launch power, the first non-linear effect that needs to be dealt with is SBS3 . In this effect, the optical signal interacts with density variations inside the optical fiber (caused by acoustic modes or temperature gradients) to create a reflected Stokes wave. This effect can add considerable noise to the signal, but is reduced significantly when the linewidth of the laser is broadened.


For direct laser modulation, typically employed in 2.7Gb/s transmission systems, the optical linewidth is inherently broadened by the adiabatic chirp induced via the RF modulation. The SBS threshold in this case is above 16dBm launch power. For externally modulated transmission (2.7Gb/s and 10.7Gb/s), SBS backreflection occurs at relatively low powers since about half of the modulated signal power remains contained within the narrowband carrier signal. The maximum launch power in this case is limited to about 11dBm, but can be easily improved by increasing the laser linewidth using a low-frequency dither of the laser bias current. Laser dithering is common and can increase the SBS threshold for Distributed Feedback (DFB) lasers to approximately 18-19dBm launch power. For other types of lasers, even if dithering is used, the SBS threshold can still be significantly lower.


The second effect to limit transmission is SPM. SPM occurs when the power modulation of the on-off keying signal, self-modulates the phase of the signal, thereby causing signal distortion. For 10.7Gb/s single-span transmission, SPM limits the launch power to roughly 21dBm, and for 2.7Gb/s, the SPM limit is well above this4 . In both cases, the maximum power in a single-span system is limited by SBS, and not by SPM.


Therefore, the use of a Power Booster in single-span applications should be considered using the following criterion:

a)For single-channel links - employ a standard booster without a Power Booster.

b)For 2-6 channels links - employ Power Booster designed with output power of 18dBm+10logN, where N is the number of channels.

c) For more channels and/or for additional link budget - employ a DRA in addition to the Power Booster, as described in the next section


........



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