R. R. G. Gamatham

Experimental characterization of the counter-propagating Raman polarization attraction.

Recently, fiber Raman amplifiers have proven to be effective in the all-optical control of the state of polarization of signals in single-mode telecommunications optical fibers. Previous works predicted the existence of a quantitative relationship between the achieved degree of polarization and the mean Raman gain. Here, we experimentally validate such a relationship in the case of counter-propagating Raman-based polarization attractors for different pump and signal powers and for different fiber link lengths.

Square Kilometre Array: The radio telescope of the XXI century

The Square Kilometre Array (SKA) will be the world’s largest and most sensitive radio telescope. It will address fundamental unanswered questions about our Universe including how the first stars and galaxies formed after the Big Bang, how dark energy is accelerating the expansion of theUniverse, the role of magnetism in the cosmos, the nature of gravity, and the search for life beyond Earth. This project envisages the construction of 133 15-m antennas in South Africa and 131072 log-periodic antennas in Australia, together with the associated infrastructure in the two desert sites. In addition, the SKA is an exemplar Big Data project, with data rates of over 10 Tbps being transported from the telescope to HPC/HTC facilities.

Fiber-to-the-telescope: MeerKAT, the South African precursor to Square Kilometre Telescope Array

Abstract. Scientific curiosity to probe the nature of the universe is pushing the boundaries of big data transport and computing for radio telescopes. MeerKAT, the South African precursor to Square Kilometre Array, has 64 antennas separated by up to 12 km. By 2018, each antenna will stream up to 160 Gbps over optical fiber to a central computing engine. The antenna digitizers require highly accurate clock signals distributed with high stability. This paper outlines requirements and key design aspects of the MeerKAT network with timing reference overlay. Fieldwork results are presented into the impact of birefringence and polarization fluctuations on clock stability.

Simultaneous 10 Gbps data and polarization-based pulse-per-second clock transmission using a single VCSEL for high-speed optical fibre access networks

Access networks based on vertical cavity surface emitting laser (VCSEL) transmitters offer alternative solution in delivering different high bandwidth, cost effective services to the customer premises. Clock and reference frequency distribution is critical for applications such as Coordinated Universal Time (UTC), GPS, banking and big data science projects. Simultaneous distribution of both data and timing signals over shared infrastructure is thus desirable. In this paper, we propose and experimentally demonstrate a novel, cost-effective technique for multi-signal modulation on a single VCSEL transmitter. Two signal types, an intensity modulated 10 Gbps data signal and a polarization-based pulse per second (PPS) clock signal are directly modulated onto a single VCSEL carrier at 1310 nm. Spectral efficiency is maximized by exploiting inherent orthogonal polarization switching of the VCSEL with changing bias in transmission of the PPS signal. A 10 Gbps VCSEL transmission with PPS over 11 km of G.652 fibre introduced a transmission penalty of 0.52 dB. The contribution of PPS to this penalty was found to be 0.08 dB.

A 1550-nm all-optical VCSEL-to-VCSEL wavelength conversion of a 8.5-Gb/s data signal and transmission over a 24.7-km fibre

For the first time, we demonstrate, VCSEL-to-VCSEL wavelength conversion within the low attenuation 1550 nm window, including transmission over fibre and bit error rate (BER) performance characterization. We experimentally demonstrate a low injection power optical wavelength conversion by injecting an optical beam from a signal carrier master vertical cavity surface-emitting laser (VCSEL) into the side-mode of the slave VCSEL. This technique solves the challenge of wavelength collisions and also provides wavelength re-use in typical wavelength division multiplexed (WDM) systems. This paper, for the first time, uses two 1550 nm VCSELs with tunability range of 3 nm for a 5-9.8 mA bias current. The master VCSEL is modulated with a non-return-to-zero (NRZ) pseudo-random binary sequence (PRBS_27-1) 8.5 Gb/s data. A data conversion penalty of 1.1 dB is realized when a 15 dBm injection beam is used. The transmission performance of the converted wavelength from the slave VCSEL is evaluated using BER measurement at a 10-9 threshold. A 0.5 dB transmission penalty of the converted wavelength data is realized in an 8.5 Gb/s transmission over 24.7 km. This work is vital for optical fibre systems that may require wavelength switching for transmission of data signals.

Effects of polarization mode dispersion (PMD) on Raman gain and PMD measurement using an optical fibre Raman amplifier

We experimentally investigate the effects of increase of the differential group delay (DGD) on Raman gain and gain value. It was found that the increase of DGD values lead to the formation and the increase in the number of ripples at the expense of gain. A Raman amplifier with gain of 6.65 dB has been experimentally designed using a 24 km-SMF fibre spool with a gain fluctuation of ±0.2dB. Measurement of PMD using an optical fibre Raman amplifier has also been investigated and validated. This work recommends low values of DGD to reduce signal and gain distortion.

Fibre-to-the-telescope: MeerKAT, the South African precursor to square kilometre telescope array (SKA)

Scientific curiosity to probe the nature of the universe is pushing boundaries of big data transport and computing for radio telescopes. MeerKAT, the South African precursor to Square Kilometre Array (SKA), has 64 antennae separated by up to 12 km. By 2018, each antenna will stream up to 160 Gbps over optical fibre to a central computing engine. The antennae digitizers require highly accurate clock signals distributed with high stability. This paper outlines requirements and key design aspects of the MeerKAT network with timing reference overlay. Fieldwork results are presented into the impact of birefringence and polarization fluctuations on clock stability.

Noise figure and pump reflection power in SMF-reach optical fibre for raman amplification

We investigate both experimentally and by simulation the Noise Figure (NF) and pump reflection power of a Distributed Fibre Raman Amplifier (DFRA) as a function of pump power and fibre length, at different pump configurations. A DFRA of lengths 25 km and 50 km has been experimentally designed using Single Mode Reach Fibre (SMF-Reach) with an attenuation of 0.20 dB/km. Two pumping techniques namely Co-pumping and counter pumping have been used. It was found that the increase in fibre length, lead to increase in the NF irrespective of the pumping technique used. A NF of -2.2 dB and -1.9 dB was achieved experimentally for Co- and Counter pumping schemes respectively, for a 25 km fibre at 23 dB pump power. For a 50 km fibre, a NF of -1.8 dB and -0.7 dB was obtained for the two pump configurations respectively, under the same pump power. Pump reflection power varied inversely with gain and directly with fibre length. This work recommends Co-pumping technique in signal transmission due to its low NF and pump reflection power.

Characterization of the polarization mode dispersion of single mode fibre in KAT-7 telescope array network, South Africa

We experimentally investigate the differential group delay (DGD) in KAT-7 optical fibre network using polametric measurement technique. Low PMD values in the femtosecond range are obtained over the 10 km single mode fibre. It is also established that deployment of the fibre alters the DGD of the network. This study is paramount in ensuring accurate and stable clock signal as well as error free transmission at very high bit rates in the optical network.

Fibre-to-the-Hut: Research into tailored FTTH solutions for Africa

Next-generation FTTH technology is aggressively being developed and deployed in Europe, Asia and in the USA. Countries like Japan with high per-capita income and high-density residential complexes remain at the forefront of FTTH and WDM-PON developments. The majority of African countries face unique socio-economic challenges, along with sprawling population distributions. As such, there is a need to develop network technologies, network architectures and business models applicable to the African case. In this presentation we discuss these issues and present potential solutions. We further show our experimental results of how low-cost Vertical Cavity Surface Emitting Lasers (VCSELs) are suited to long-reach 4.25 Gbps access networks in the African context. This work is part of a new and exciting project “Fibre-to-the-Hut: Researching Novel FTTH Solutions for Africa”, currently being launched by the Nelson Mandela Metropolitan University.