A. W. R. Leitch

Statistical characterization of the output state of polarization in deployed single-mode fibers

Changes in the optical fiber properties due to both intrinsic and extrinsic variations result in polarization mode dispersion and state of polarization (SOP) becoming stochastic in nature. The statistics for first-order PMD and the second-order PMD approach the Maxwellian and Foschini et al. [IEEE Photonics Technol. Lett.12, 293 (2000)] distributions, respectively. In this Letter, we investigate a theoretical statistical distribution that corresponds to output SOP variations. The SOP variations can either be with wavelength (for buried fiber) or with time (for aerial fiber). Our results show that the statistics of the relative SOP changes approach the distribution proposed by Foschini et al..

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.

Single-End Measurement of Root Mean Square Differential Group Delay in Single-Mode Fibers by Polarization Optical Time-Domain Reflectometry

We present a novel technique, based on polarization-sensitive reflectometry, able to perform single-end measurements of the root mean square differential group delay of single-mode fibers. The technique uses single wavelength measurements and may be applied to sections of fiber links. The theoretical analysis is supported by experimental results.

Polarization mode dispersion emulation using polarization maintaining fibers: Fixed root-mean-square differential group delay but varying second-order polarization mode dispersion

We report on a polarization mode dispersion (PMD) emulator with a fixed root-mean-square differential group delay (RMS-DGD) but varying second-order PMD (SO-PMD) using only a combination of polarization maintaining fibers and a polarization controller. The SO-PMD control mechanism is not completely in real time. Besides controlling the mean PMD values of the emulator, simultaneous adjustments in the maximum and minimum values of PMD statistics can be performed. We therefore illustrate irregular fluctuations that occur around the RMS-DGD due to SO-PMD. This novel design can be used to further show the impact of a high first-order PMD segment on the DGD and SO-PMD statistical distributions that might occur in an optical network system.

Locating high PMD sections of an overhead cable using polarization-OTDR

High Polarization Mode Dispersion (PMD) values pose an obstacle to the upgrading of links for higher transmission rates. Polarization Optical Time Domain Reflectometry (P-OTDR) measurements give an indication of the specific fiber sections that cause the high PMD values, allowing an informed decision to be taken regarding the upgrading of a link. In this work P-OTDR measurements are used to indicate the fiber sections potentially giving rise to high PMD in an overhead optic fiber cable in the Eastern Cape, South Africa. Verification of the P-OTDR predictions is done by cutting out the sections flagged as being high PMD contributors, and measuring actual PMD values.

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.

Capacity upgrade in short-reach optical fibre networks: simultaneous 4-PAM 20 Gbps data and polarization-modulated PPS clock signal using a single VCSEL carrier

Abstract In this work, a four-level pulse amplitude modulation (4-PAM) format with a polarization-modulated pulse per second (PPS) clock signal using a single vertical cavity surface emitting laser (VCSEL) carrier is for the first time experimentally demonstrated. We propose uncomplex alternative technique for increasing capacity and flexibility in short-reach optical communication links through multi-signal modulation onto a single VCSEL carrier. A 20 Gbps 4-PAM data signal is directly modulated onto a single mode 10 GHz bandwidth VCSEL carrier at 1310 nm, therefore, doubling the network bit rate. Carrier spectral efficiency is further maximized by exploiting the inherent orthogonal polarization switching of the VCSEL carrier with changing bias in transmission of a PPS clock signal. We, therefore, simultaneously transmit a 20 Gbps 4-PAM data signal and a polarization-based PPS clock signal using a single VCSEL carrier. It is the first time a signal VCSEL carrier is reported to simultaneously transmit a directly modulated 20 Gbps 4-PAM data signal and a polarization-based PPS clock signal. We further demonstrate on the design of a software-defined digital signal processing (DSP)-assisted receiver as an alternative to costly receiver hardware. Experimental results show that a 3.21 km fibre transmission with simultaneous 20 Gbps 4-PAM data signal and polarization-based PPS clock signal introduced a penalty of 3.76 dB. The contribution of polarization-based PPS clock signal to this penalty was found out to be 0.41 dB. Simultaneous distribution of data and timing clock signals over shared network infrastructure significantly increases the aggregated data rate at different optical network units (ONUs), without costly investment.

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.