Paper Title (use style: paper title)

Enhancing Optical Signal-to-Noise Ratio in Terrestrial Cascaded EDFAs Fiber Optic Communication Links using Hybrid Fiber Amplifier

Abstract -In this paper, we propose calculating models of Terrestrial DWDM cascaded EDFAs Fiber Optic Communication Links using Hybird Fiber Amplifier (HFA) including combination of Distributed Raman Amplifier (DRA) and EDFA, and build algorithm charts to optimize parameters including signal power per channel launched fiber, EDFAs gain, pump power of Raman amplifier for improving optical Signal-to-Noise ratio (OSNR) at the end of link. Then, algorithm-based numerical calculating (MathCAD program) is applied in typical system (WDM Nation-wide links in Vietnam). Optimized results show that the OSNR can be increased by (1-5)dB comparing with that in cases of parameters chosen by experience way.

Keywords: OSNR, Terrestrial; WDM; cascaded EDFA; Hybird Fiber AmplifierI. Introduction Unlike Long-Haul undersea fiber optic communication systems using in-line EDFAs chains where every span is located equally, Nation-Wide backbone terrestrial systems are connected through many cities that the distances between among them are quite different. In such as systems, EDFAs are usually located at main communication centers in these cities to be convenient for system management, control and maintenance.Therefore, spans have different lengths depended on cities positions. In practice, after installing all system components and configuration, the parameters of equipments such as optical transmitters, all EDFA gains are chosen basing on manufactures recommendations and/or compensating completely the transmission loss of preceded span. In general, recommended values of parameters are varied in broad band, In that case, system can not be set up their optimized values to achieve a high OSNR at the end of transmission link.

Yet, many WDM cascaded EDFAs systems using HFA have been investigated, experimented and deployed in order to get better OSNR. Through theory and numerical studies, they have been demonstrated to be able to expand bandwidth from C to L band and enhance OSNR [12], [13], [14], [15]. OSNR improving ability of HFA (combinating of Distributed Raman Amplifier DRA and EDFA) compared with original transmission (using EDFA) could be explained as follow: DRA which is located at the end of span (fig.2,3,4.) is considered a pre-amplifier of EDFA. It makes signal power level at the end of span higher and EDFA Gain smaller (see fig.1.) Therefore, nonlinear effects, different noises in fiber could be reduced. This results in higher OSNR at the end of link.

In these systems, however, if values of signal power launched fiber, pump power of raman amplifier and EDFAs gain have chosen basing on manufactures recommendations or by experience, each individual equipment and components can operate indifferently well. That operation may be locally optimized, not globally optimized because the interactions and co-ordinations of individual components for the whole link have not been considered yet. This means that system can not be set up their optimized values to achieve a higher OSNR at the end of link. In this paper, from calculating models (fig.2,3,4.), we build algorithm charts for optimizing OSNR at the end of link. Numerical calculating then, is applied in typical system (WDM Nation-wide Links in Vietnam) to get higher OSNR by (1-5)dB comparing with that in links where parameters are chosen conventionally by experience way.

The rest of paper is organized as follows. In section II, We propose three typical calculating models, then, present calculation expressions signal power, dominant different noises, OSNR at the end of link. In section III, we build algorithm charts for calculating total accumulated ASE, FWM, DRS noise powers, and for optimizing OSNR in general case for all three calculating models (fig 2.,3.,4.). In section IV, we apply numerical calculating in WDM Nation-wide Links in Vietnam then analyze the results. Section V will be our conclusion.

II. CALCULATING MODELS

When distance of Terrestrial WDM cascaded EDFAs Fiber Optic Comminication Link is shorter than 1,000km, we propose its calculating model consisting only one HFA in order to decrease reasonably its cost and complication.

Hence, three typical calculating models are given as in Fig.2,3,4 in which HFAs are located in the first span, middle span and last span respectively.

They are considered as extension of model proposed in [1] to calculate HFA. Where: M; M-1: the number of spans and EDFAs in the Link; L(m) (m = 1..M) : mth span length

Gi(m), Gj(m), Gk(m) : gain of the mth EDFA at channel i, j, k respectively Pi, Pj, Pk : input power corresponding to channels i, j, k Pij k(m) : Four-Wave Mixing (FWM) noise power that is caused by FWM effect at the end of transmission link created within the sections mth Fig. 5. shows configuration of HFA with Backward pump at 1450nm.

+ Optical Signal-to-Noise Ratio (OSNR) at the end of transmission link

In this paper, total optical power launched fiber is limited less than 20dBm. Thus, the other effects of fiber nonlinearities (SPM, XPM, SBS, SRS) are significantly less than FWM one and can be neglected[1]-[4]. As a result, Optical Signal to Noise Ratio (OSNR) at the end of link, respectively mth channel (fm) is shown as:

(1)

where: : is signal power at the end of link

, and are total FWM, accumulated ASE and Double Raman Scattering noise powers at optical mth channel, respectively.

+ Signal power at the end of link

In a Fiber Optic Communication Link using cascaded EDFAs and HFA, signal power received at the end of link Psig(fm) is amplified by in-line EDFAs and is also decreased by attenuation of fiber in spans. Therefore, it is presented as follows

(2)

where:

Pin(fm) : transmitted signal power assumed that are equally for all channels

the ith EDFA at mth channel; : Gain of Raman ; 2A(i): attenuation in ith span

+ FWM noise at the end of link

As system has a large number of channels, at a certain frequency fm, a significant amount of FWM waves is created by various combinations of channels in system[1],[5]. As a result, the total FWM power at fm is shown as

(3)

: FWM noise power that is created by all M spans causes at the end of link[1]:

EMBED Equation.3 (4) + Accumulated ASE noise at the end of linkWe investigate total Accumulated ASE noise for general case (can be applied in all three general models in fig.1, fig.2 and fig.3) in which Raman amplifier is located in kth span, 2