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ofc.2013.om3h.3

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OM3H.3.pdf
OFC/NFOEC Technical Digest © 2013 OSA
Digital Multi-Wavelength Generation and Real Time Video
Transmission in a Coherent Ultra Dense WDM PON
H. Rohde1, E. Gottwald1, Pedro Alves2, Carlos Oliveira2, I. Dedic3, T. Drenski4
1
Nokia Siemens Networks Optical, St. Martin Str. 76, 81541 Munich, Germany
2
Nokia Siemens Networks, Rua Irmaos Siemens 1, Amadora, Portugal
Fujitsu Semiconductor Europe GmbH, 3 Concorde Park, Concorde Road, SL6 4FJ Maidenhead, Berkshire, United Kingdom
4
Fujitsu Semiconductor Europe GmbH, Pittlerstrasse 47, 63225 Langen, Germany
{Harald.Rohde, Erich.Gottwald, Pedro.Alves, Carlos.Oliveira}@nsn.com, ian.dedic@uk.fujitsu.com, tomislav.drenski@de.fujitsu.com
3
Abstract: Generation of multiple individually modulated wavelengths out of a single laser source
is one of the key pre-requisites for an economical UDWDM PON. It is implemented using a 65
GS/s DAC and real time DSP.
OCIS codes: (060.1660) Coherent communications, (060.4080) Modulation
1. Introduction
Coherent Ultra-Dense WDM (UDWDM) PONs have been described by several authors [1-3]. Detailed
implementations differ, but all UDWDM PONs have a very dense WDM spacing in the order of 3 GHz. Such a
dense wavelength spacing can be generated by optical combs (with the challenging task to separate the single comb
lines and modulate them individually) or by adding spectrally closely spaced lasers together. In the latter case, the
challenge is to stabilize the frequency offset precisely in a cost effective manner.
However, current DAC and ADC cores have sampling rates of up to 65 GS/s and 6 effective bits [4], enabling
analog frequencies of up to 32 GHz to be generated and detected. Using this broad frequency range and employing
IQ full optical field modulation techniques, an optical spectral range of up to ±30 GHz can be modulated. This
enables the generation of up to about 20 modulated carriers out of a single laser source with sufficient carrier to
noise ratio.. This paper presents an experimental set-up of such a system and some measurement results.
2. Experimental Set Up
The system described in this paper generates 10 wavelengths out of a single laser source. Each wavelength is a
DQPSK modulated real time 1.244 Gbit/s data stream, modulated independently from all other wavelengths. The
optical frequencies are {-14; -11; -8; -5; -2; 2; 5; 8; 11; 14}*933.12 MHz relative to the optical carrier. Figure 1
shows the setup of the transmitter. An Ethernet switch chip assigns Ethernet frames to the respective wavelengths
and forwards the frames to two FPGAs, each responsible for 5 of the wavelengths. The FPGAs map the Ethernet
frames into ODU 0 frames and add FEC and additional TC layer information. Each FPGA has a 10 Gb/s link into
the ASIC which transmits the bit streams for 5 wavelengths. The ASIC consists of the following building blocks:
The first section demultiplexes the two 10 G b/s bit streams from the FPGAs for each of the 10 wavelengths into an
I and Q bit pair stream for the differential PSK modulation. The next stage contains for each wavelength an upconversion and pulse shaping unit which brings the base band I-Q bit pair streams to the respective frequencies. It
also performs a programmable pulse shaping in order to limit the bandwidth of the single modulated carriers to the
assigned wavelength band. A good spectral confinement of the generated wavelength to their assigned bands by
proper pulse shaping is essential to avoid interference with the upstream data from the ONUs, which is spectrally
placed in-between the downstream wavelengths. More details on the frequency plan can be found in [2]. The digital
data from all 10 wavelengths is then added for both the I- and the Q- branch and sent to the 56 GS/s I- and Q-DACs.
As a final step, the output of the two DACs is handed to an IQ modulator which generates the optical signal by
modulating the light of a tunable laser with low linewidth.
The ONU, as described in [2], consists of a tunable laser and a coherent heterodyne receiver. The light from the
tunable laser is used both for upstream data transmission and for heterodyne coherent reception. The ONU operates
by locking to an assigned downstream wavelength and by transmitting upstream with an offset of 933.12 MHz.
For diagnostics a high resolution OSA is connected to the connecting link between OLT and ONU such that the
OSA gets its input from both downstream and upstream direction.
In order to demonstrate the real time capabilities, a video server delivers a HD video stream through the OLT
and the ONU to a video client which shows the HD video content. The system described in this paper just serves as
a transparent bit pipe.
978-1-55752-962-6/13/$31.00 ©2013 Optical Society of America
OM3H.3.pdf
OFC/NFOEC Technical Digest © 2013 OSA
Fig. 1: Transmitter setup
3. Results
To explain the principle, fig. 2 shows the optical spectrum on the fiber, upstream and downstream combined, for
only one downstream wavelength and for one ONU which is locked to that respective wavelength. The payload
wavelength is spectrally located at -1866 MHz relative to the laser source. A pulse shaping filter with a square root
raised cosine characteristic with α=0.5 is applied and its effect can clearly be seen due to the absence of spectral
pedestals. The spectral mirror of the payload wavelength can be seen at +1866 MHz with a single sideband
suppression of about 20 dB in this case. The wavelength plan of the OLT is symmetric, and when another
wavelength is put onto the +1866 MHz frequency slot, this wavelength is superimposed with the residual sideband
of the -1866 MHz wavelength. However, as the difference in strength is 20 dB, the residual second sideband does
not disturb data transmission on the +1866 MHz wavelength. Two lines at ±933 MHz can be seen as well; those are
pilot tones which are used for the automatic bias algorithm to tune the nested MZM based IQ modulator to its right
bias points. The central frequency is suppressed, but the imperfect extinction ratio of the IQ modulator leaves a
residual central frequency signal. Please note that although the amplitudes of the pilot tones and the residual carrier
in fig. 2 have a similar level to the payload signal, the integrated total power of these signals is much less than the
integrated optical power of the payload wavelength. The peak at -2799 MHz shows the position of the ONU local
oscillator laser. It has a spectral distance of 933 MHz with regard to the payload wavelength at -1866 MHz, resulting
in a 933 MHz heterodyne signal at the coherent detector. The central dips in the four narrowband signals are
artifacts due to the measurement instrument and not real features of the signals.
Fig. 2: Optical spectrum, upstream and downstream light added
OM3H.3.pdf
OFC/NFOEC Technical Digest © 2013 OSA
Bit error rate curves have not been measured, because the ONU local oscillator laser used in the set-up suffers from
a high noise floor due to internal reflections. The BER is about 10-6 for a wide range of optical input powers. A
sensitivity of -53 dBm at BER=10-3 has been reached [2]. However, the BER is sufficiently low such that error free
post FEC HD video transmission is possible over the system.
Fig. 3 shows the optical spectrum in downstream direction for the complete system with different wavelength
configurations. The center frequencies of the wavelengths follow the frequency grid as stated in the experimental
setup section. In the current implementation the layout of the printed circuit board was not optimized with regard to
a flat radio frequency transmission for up to 15 GHz and therefore the wavelength with higher frequencies are quite
strongly attenuated. Especially the wavelengths at ±7.5 GHz suffer from a non-flat RF frequency response of the
board.
Fig. 3: Optical spectrum with a) 5 downstream wavelengths and b) 10 wavelengths
In fig 3a) only the 5 wavelengths with the negative frequencies are switched on. Good side band suppression of
15 to 20 dB can be observed. Better sideband suppression will be achieved by tuning the amplitudes and phases of
all wavelengths individually, but even the current sideband suppression is sufficient to insure good transmission
quality in the 10 wavelength case, as shown in fig. 3b) where all wavelengths are active.
4. Real time HD video transmission
For demonstration purposes a video server was connected to the Ethernet switch in the transmitter, which sends
gigabit Ethernet frames. A real time High Definition video stream was mapped into the Ethernet payload, decoded at
the ONU side and then displayed by a video client. To the best knowledge of the authors, this was the first time that
real time video traffic was send over a coherent ultra dense WDM PON with digital multi-wavelength generation.
5. Summary and Outlook
Digital multi-wavelength generation and real time video transmission in a coherent ultra dense WDM PON has been
successfully demonstrated, using a dual 65 GS/s, 8 bit DAC in combination with a small DSP core for upconversion and pulse shaping on the same ASIC. In the next steps the residual sideband suppression will be
improved by individually tuning each wavelengths electrical amplitude and phase. The amplitude uniformity will be
improved by a new electronics board, together with digital equalization. Sensitivity will be further improved by
more advanced DSP algorithms like e.g. phase noise cancellation techniques [5].
5. References
[1] S. Smolorz, E. Gottwald, H. Rohde, D. Smith, A. Poustie, “Demonstration of a Coherent UDWDM-PON with Real-Time Processing”,
PDPD4, OFC 2011
[2] H. Rohde, S. Smolorz, J. S. Wey, E. Gottwald, “Coherent Optical Access Networks”, OFC 2011, OTuB1
[3] J. Prat, V. Polo, P. Zakynthinos, I. Cano, J. A. Tabares, J. M. Fàbrega, D. Klonidis, I. Tomkos,” Simple Intradyne PSK System for udWDMPON”, We.2.B.2, ECOC 2012
[4] I. Dedic, “56Gs/s ADC Enabling 100GbE”, OFC 2010, OthT6
[5] S.Y. Kim, N. Sakurai, H. Kimura, H. Hadama, “VCSEL based Coherent Detection of 10 Gbit/s QPSK Signals Using Digital Phase Noise
Cancellation for Future Optical Assess Systems”, OFC 2010, OMK6
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