3 July 2014
Dr Hirotaka Ono of NTT Photonics Laboratories tells us about the research behind the Letter ‘Inter-core crosstalk measurement in multicore fibre amplifier by using multiple intensity tones’
I have been engaged in the research and development of optical fibre communication technologies for both long-haul and metro networks. The technologies include optical fibre amplifiers, highly nonlinear fibre devices, photonic crystal fibres, and tuneable optical dispersion compensators. Since 2011, I have mainly been working on the research of lumped and distributed optical amplification technologies needed for constructing long-haul space-division-multiplexing (SDM) transport systems.
SDM is designed to increase transmission capacity using a multi-core fibre (MCF), and/or a few mode fibre (FMF), and sometimes including the parallel use of single-core fibre. Recent progress on communication technologies and services has led to rapid growth of internet traffic, and the demand for high capacity optical transport networks is expected to continue over the coming years. The capacity of optical transport systems has grown through incorporation of WDM, multi-level modulation formats, and digital signal processing technologies. However, the transmission capacity will eventually reach its limit of around 100 Tbit/s per fibre, because a high capacity requires a high SNR, and SNR improvement is limited by the optical nonlinear effect in transmission fibres and the fibre fuse phenomenon. An SDM technique is expected to overcome this problem.
Crosstalk between the cores of an MCF, called inter-core crosstalk, is a specific issue with SDM transmission employing MCF as a transmission fibre. Leaked light from other cores, caused by mode coupling between cores, becomes crosstalk in the core, and this crosstalk light may degrade the transmission characteristics. For example, it has been reported that high bit rate modulation formats, such as PDM-64 QAM and PDM-128QAM, require crosstalk values below −30 dB to achieve a Q-penalty of 1 dB. Therefore, the crosstalk value must be characterised and improved if we are to employ an MCF and a multicore erbium-doped fibre amplifier (MC-EDFA) in a practical transmission system with sufficient crosstalk margin.
We have described our investigation of a method for measuring the inter-core crosstalk in an MC-EDFA, which employs a single-wavelength signal with multiple intensity tones. The basic concept of the measurement was previously proposed by one of the co-authors as a way of evaluating the modal crosstalk in an FMF. We use the concept as a basis for evaluating the inter-core crosstalk of an MC-EDFA and clarifying its feasibility.
We need to characterise the inter-core crosstalk between signals at the same wavelength propagating in different cores. However, it is difficult to measure crosstalk in the optical domain, because the signals cannot be distinguished in a power measurement with an optical spectrum analyser. Therefore, inter-core crosstalk has been measured by using different wavelength signals. The most significant advantage of our method is that it can evaluate the inter-core crosstalk of an MC-EDFA by using the same-wavelength signals between different cores. Our method also has extensibility, in that multiple crosstalk values can be evaluated in a single measurement by employing a number of tones with different modulation frequencies.
Certain challenges remain to be overcome before our method can be put to practical use. One is that inter-core crosstalk measurement in an MC-EDFA suffers from amplified spontaneous emission, especially when the input signal power is small, and then the optical SNR of the output signal of the MC-EDFA is small. We must undertake further study of the dynamic range of our method for different input signal powers before it can be employed practically.
This work was partly supported by a national project on the R&D of an innovative infrastructure for optical communications, initiated by the Extremely Advanced Transmission (EXAT) technology study group and funded by the National Institute of Information and Communications Technology (NICT), Japan. The project is being undertaken in collaboration with another national project on the R&D of innovative optical fibres. These projects are working on the development of key elements for SDM systems, such as MCFs and FMFs, amplifiers and connection technologies, and performing system evaluations by combining them. We are currently working on SDM amplification technologies as part of the project, and applying the techniques to an SDM long-haul transmission experiment.
This interview is based on the letter 'Inter-core crosstalk measurement in multi-core fibre amplifier using multiple intensity tones' (new window)
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