Arbitrary high speed qam optical transmitter

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Arbitrary High Speed QAM Optical Transmitter
using FPGA and 34GBd DAC
Vitor B. Ribeiro, Luis P. F. de Barros, Luis H. H. de Carvalho, Anderson C. Bravalheri, Luis R. Monte, Pedro P. G.
Cardoso and Julio C. R. F. Oliveira
CPqD Foundation, Optical Communications Systems, Photonics Division, Campinas, Brazil
{vribeiro, lbarros, lhecker, abraval, lmonte, julioc}@cpqd.com.br

Abstract— In thispaper we present a high speed
N-QAM transmitter capable of generating constellations up to
25GBd using a high-speed DAC and FPGA. Electrical generation
of 4-QAM, 16-QAM and 64-QAM is achieved with proposed
DAC/FPGA structure. A high quality optical generation of
DP-QPSK (4-QAM) and DP-16QAM signal is demonstrated. The
main issues related to electrical and optical generation are
discussedand solutions for the common pitfalls are outlined.
Keywords-component; Optical Communications; Quadrature
Amplitude Modulation; High-Speed Digital to Analog Converter;
Digital Signal Processing.

I.

INTRODUCTION

There has been significant interest in high speed electronic
digital signal processing (DSP), in recent years, for overcoming
transmission impairments in coherent opticalsystems.
Chromatic dispersion (CD), polarization mode dispersion
(PMD), frequency offset (FO) and laser phase noise can be
mitigated using digital filters [1-2]. Those techniques are
usually implemented in the receiver side and improved system
capacity not only by impairment mitigation, but also for
handling well polarization multiplexing (PM) and high order
modulation formats [3]. In order tofurther improve system
capacity the DSP is being used in the transmitter side [4-7].
Pre-filtering the signal makes possible to increase spectral
efficiency (SE) by applying orthogonal frequency division
multiplexing (OFDM) [5], Nyquist filtering [4,6] and other
pulse shaping techniques [7]. Moreover the transmitter can
generate arbitrary N-quadrature amplitude modulation (NQAM)constellations depending on the requirements and link
conditions [8]. Commercial or practical applications entail that
transmitters and receivers execute real-time data processing.
Field programmable gate arrays (FPGA) or application specific
integrated circuits (ASIC) coupled with high-speed digital-toanalog (DAC) allow the realization of real-time 100 Gbit/s and
beyond optical transmitters, aiming forthe next generation of
terabit SE transponders [5].
In this paper is presented an optical transmitter capable of
generating arbitrary PM-QAM constellations up to 25GBd. The
simplified functional block diagram of the transmitter is
illustrated in Fig. 1. The FPGA data generation structure and its
synchronization with the DAC are presented and the common
drawbacks presented. Also, will bepresented necessary
adjustments to linearly modulate optical signals considering
electrical driver and modulator transfer functions.

II.

TRANSMITTER STRUCTURE

The transmitter consists basically on the functional blocks
presented at Fig. 1. The clock source for the electronics works
at half of the baud rate of the DAC; therefore to achieve
25GBd the clock frequency was set to 12.5GHz.Considering
only the DAC limitations the baud rate could be as high as
34GBd, however FPGA high speed interfaces limits overall
baud rate to 25GBd. FPGA generate data words to drive the
DAC; all of them synchronized with reference clock of
156.25MHz provided by DAC. Afterwards the DAC analog
outputs are fed to RF amplifiers, deco-related and then fed to
IQ-Modulator, for generating the opticalsignal. Subsequently
the transmitter implementation and functional blocks will be
described in more detail.

Fig. 1. Transmitter funcional blocks.

A. High-Speed DAC
Fig. 2 illustrates the high-speed DAC in more detail. The
6-bit Micram VEGA DAC II is capable of delivering up to
34GS/s. The data to be converted to analog are transmitted
through SerDes ports, which run at the output...
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