RICERCA ITALIANA     

Transmission of Optical Signals exploiting Competitive Amplification techniques (TOSCA)

Scuola Superiore di Studi Universitari e Perfezionamento S. Anna di Pisa

Abstract

Since Nineties, optical transmission systems experienced impressive developments, which enabled the deployment of the Internet. This has been enabled by the EDFA (Erbium Doped Fibre Amplifier) technology: the EDFA amplifier indeed supported a remarkable increase of system performance, as it allowed for quite longer fibre span and, moreover, for a massive capacity increase (being compatible with the Wavelength Division Multiplexing, WDM).

The following traffic increase showed that today network is far from being optimised. First, the protocol stack sets a relevant burden. On the other hand, the optical transport equipment is expensive, complex and, to some extent, too large.

Hence, new system architectures are urgently needed, with greater efficiency and lower cost (as an example, significant research efforts is devoted to obtain optical components with much lower size).

Namely, the EDFA is probably the most critical component, as in a single link many EDFAs are deployed, each including several components (one or two pump lasers, optical isolators, various meters of active fiber).
To this aim, SOAs (Semiconductor Optical Amplifier) might be considered as a replacement for EDFAs, thanks to their small size, low power consumption, and the potentially competitive cost. However, SOAs are not compatible with the always-used IM (Intensity Modulation, or On-Off Keying, OOK): this is the main reason why EDFAs were preferred. Today >>>

Principal Investigator

Ernesto CIARAMELLA Scuola Sup. di Studi Univ. e Perfezionamento S.Anna di PISA

Research Objectives

TOSCA project is aimed to study and experimentally assess new system solutions based on SOAs (Semiconductor Optical Amplifiers) and CE (Constant Envelope) modulation formats. EDFAs are widely preferred to SOAs mainly because EDFA gain is not affected by the fast intensity modulation of optical input signals. On the other hand, the population inversion in SOAs rapidly changes when the input intensity is modulated as in current IM-based Gbit/s systems. Hence, if a single channel is injected into a SOA, the output can be affected by the so-called pattern effect; even worse, in the WDM case, each channel may suffer from Cross Gain Modulation (XGM) effect due to the other channels. The easiest way to suppress XGM is to operate the SOA with input power lower than the saturation level ("nearly linear regime"). Yet, this may severely impair the optical signal to noise ratio (OSNR), so that around 40 km amplifier span would have to be used (quite shorter than the 80 km span usual in EDFA-based systems).

Recently, gain clamped (GC) SOA and Linear Optical Amplifiers (LOAs) were demonstrated: in these special SOAs the optical gain is forced to be constant, thus avoiding the XGM effect. This is achieved by a particular design of the component, e.g. in the LOA vertical cavity lasers (VCSELs) are used to saturate the optical gain, which is than far less affected by any input intensity variation. This scheme, however, has some drawbacks: as pointed out by some papers, this >>>

First Results

In the following we will briefly list the most significant expected results for each task of WP1
- T.1.1: implementation of innovative solutions for DPSK RX
- T.1.2: realise an automatic polarization controller for POLSK RX
- T.1.3: innovative solutions for CPFSK TXIn the following we will briefly list the most significant expected results for each task of WP2
- T.2.1: SOA accurate model
- T.2.2: modelling XGM-impairments in CE signals with copropagating Raman amplification
- T.2.3: analitical evaluation of using CE formats with SOAs
- T.2.4: comparativa analysis of CE formats for SOA applicationsIn the following we will briefly list the most significant expected results for each task of WP3

- T.3.1: compare different TX/RX performance
- T.3.2: experiments with Nx10 Gbit/s systems, possibly outperforming present record values for SOA-based systems, using either unidirectional or bidirectional trasmission;
- T.3.3: experiments on a single span using bidirectional Raman pumping, possibly outperforming present record values for SOA-based transmission;
- T.3.4: comparative analysis of IM modulation limitations;
- T.3.5: Nx10 Gbit/s multi-span experiments using inline SOAs: also in this case, better performance than in IM-based results are expected;
- T.3.6: WDM-CE system assessment in a field trial using SOAs: this activity could be a completely new results

Timescale

24 months

National and international background

TOSCA project aims at pursuing research objectives of international level, in a national scenario that is still not enough represented in optical transmission systems.

The three TOSCA Units have a long-term research experience in optical communications, which the participation to EU-funded initiatives further manifests (today, two Units are partner in ePHOTON/ONE Network of Excellence and one Unit is partner in NOBEL Integrated Project).

The three Units have gained a high level role in the national scenario of optical communications, and the existing competence is particularly strong for the specific TOSCA research lines:
1) CE (Constant Envelope)transmitter and receiver pairs: POLITO e ROMA are very active in this field since the first proposals of coherent optical communications [1]-[7], both at the theoretical and at the experimental level;:
2) SOA (Semiconductor Optical Amplifiers) applications: personnel from both PISA and ROMA have significant experience in SOA modelling and experimental applications [8]-[11];
3) WDM transmission experiments: PISA and POLITO have many-years experience in numerical simulations of optical transmission systems [12-15], in system design [16, 17] and in transmission experiments [18-21], even in field trial experiments in EU-projects and in the national fibre network.

ROMA and PISA have also a research activity in simulations and experimental use of SOA [22]-[23] and Raman >>>