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JW2A.72.pdf
OFC/NFOEC Technical Digest © 2013 OSA
Experimental Demonstration of a New Pilot Tone
Generation Method
1
Markus Roppelt1,2, Mirko Lawin1, Michael Eiselt1
ADVA SE Optical Networking, Märzenquelle 1-3, 98617 Meiningen, Germany
Lehrstuhl für Nachrichtentechnik, Technische Universität München, 80290 München, Germany
mroppelt@advaoptical.com
2
Abstract: We experimentally demonstrate a line encoding based pilot tone generation method and
compare it to conventional schemes. The proposed method leaves intact the high speed signal
quality and can be used with standard optical transceivers.
OCIS codes: (060.4510) Optical communications; (060.2360) Fiber optics links and subsystems
1. Introduction
Pilot tones are widely used in today’s fiber-optic networks for monitoring parameters of a WDM system. In most
applications, low bit rate information is encoded on top of a high speed data modulation to provide an easy method
for optical path monitoring. The advantage here is that instead of demodulation of the high speed signal only
detection and processing of the narrow bandwidth pilot tone is required, which is less complex and thus costeffective. Ref. [1] provides an overview of pilot tone applications and generation schemes.
Two methods for generating pilot tones are widely used: adding the pilot tone current to the laser bias current or
using an external variable optical attenuator, see Figures 1a and 1b. In both methods normally a small sinusoidal
signal is superimposed to the normal data modulation. However, this superposition affects a closure of the eye
opening at the receiver. In Ref. [2], the degradation to the receiver sensitivity is evaluated, both in simulation and
with experimental results. In general it can be observed that for a tone amplitude modulation index up to 10% and
for a pilot tone frequency up to 10% of the data symbol rate the receiver sensitivity is reduced by approximately
0.5 dB [2]. In the following, we will describe a new method for generation of a pilot tone by changing the encoding
scheme of the transmitted data, which does not incur the high data rate sensitivity penalty.
2. New pilot tone generation method
When the pilot tone is used for optical path monitoring or low speed overhead data transport, the optical signal is
detected by a low bandwidth photo diode. The average power of the data signal has to vary so that after filtering
with a low-pass electrical filter the pilot tone can be detected. In most applications, prior to transmission the digital
data is encoded or scrambled to yield a signal with a constant average power. I.e. in several protocols, 8b/10b coding
is employed, where an 8-bit data word is encoded into a 10-bit transmission word, which has a minimum of 4 “ones”
and a maximum of 6 “ones”. The deviation from the ideal average of 5 “ones” of a single 10-bit transmission word
is equalized by encoding the following 8-bit data word in the proper 10-bit transmission word, resulting in a
maximum running disparity (RD) of +/- 1. The 10-bit data word is either encoded using the negative or positive
running disparity (RD- / RD+) table, see also [3] for the first 8b/10b transmission code standard. We propose a tone
modulation method that modifies the momentary average power of the optical signal by varying the distribution of
ones and zeros in the data stream. This can be done by selecting the 10-bit code word from the proper 8b/10b
encoding table. For instance, if the target power is at 0.55 (relative to the peak signal power), all 10b-words in a
frame of several (e.g. 16) words are taken from the RD- table, resulting in all 10-bit code words having 5 or 6
“ones”. If the average power value to be achieved is at 0.45, the 10b-code words are taken from the RD+ table, such
that all 10-bit words have 4 or 5 “ones”. Values between 0.45 and 0.55 are achieved by using fixed (RD- or RD+)
tables for a fraction of the frames, while the remaining fraction is encoded using the standard 8b/10b encoding
process. By using this method, nearly arbitrary time evolution of the average power can be impressed onto the
optical signal, for instance sinusoidal tones.
At the receiver, the 8b/10b encoded words can be decoded using standard 8b/10b decoding tables. While disparity
errors will occur, the resulting 8-bit data word is still correct. It shall be pointed out that this new method can be
applied using standard optical transceivers (e.g. SFPs). As the pilot tone is generated in the line encoder logic, no
changes to the optical hardware are required.
978-1-55752-962-6/13/$31.00 ©2013 Optical Society of America
JW2A.72.pdf
Fig. 1a. Pilot tone modulation at the laser bias
Fig. 1b. Pilot tone modulation with an external
VOA
OFC/NFOEC Technical Digest © 2013 OSA
Fig. 1c. Pilot tone modulation using a
line encoding
3. Properties and FPGA implementation
The new pilot tone generation method needs to be compared to the conventional schemes. With the methods shown
in Fig.s 1a and 1b, the tone modulation leads to a closure of the received high-speed eye. With the novel technique,
the average power of the optical signal is changed by modifying the densities of “ones” and “zeros”, keeping the
power levels of ones and zeros constant. With a sufficiently low lower frequency cut-off, the high speed data
receiver doesn’t see this slow moving average and the new method should perform as well as a data transmission
without a pilot tone.
In order to experimentally validate the performance of the new generation method, an algorithm was implemented
on a Field Programmable Gate Array (FPGA) to generate a sinusoidal pilot tone in the range between 0.1 and 1.1
MHz. The incoming high speed (1 Gb/s) data stream is segmented into slots of 16 8b-words, which are encoded to
yield a sample of the tone signal to be generated. The equivalent sample rate of the tone signal is therefore 7.8 MHz.
For each of the slots, the 8b/10b encoding algorithm selects the encoding table such that the accumulated disparity in
the slot corresponds to the target value of the sampled sinusoidal tone signal. As the incoming data are random, the
target disparities cannot always be achieved within the slots. This can result in a higher noise level of the tone
signal.
Fig. 2. Experimental setup
4. Experimental results
The proposed method was experimentally compared to the conventional bias modulation and external VOA variants.
With the setup shown in Figure 2, all methods could be realized to compare the results under similar conditions. A
data stream (CRPAT – Compliant Random Pattern) was generated by an Ethernet protocol tester and sent to the
FPGA board. To demonstrate the new pilot modulation scheme, the modified 8b/10b encoding was applied, while
for the other two modulation schemes standard 8b/10b encoding was performed in the FPGA. The output of the
FPGA was used to drive a Mach Zehnder Modulator (MZM). Alternatively, instead of the modified 8b/10b
encoding, either the bias of the transmitter laser was modulated or the VOA input was driven using a Function
Waveform Generator. The VOA was a single-channel silicon VOA from Kotura. A fraction of the modulated optical
signal was received by a low-bandwidth photo diode and observed on an electrical spectrum analyzer. The optical
signal was attenuated, received in a standard SFP receiver and sent to the FPGA, where the modified 8b/10b
encoding was removed to meet the standard input expectation of the protocol tester. This step was needed as the
Ethernet protocol tester would interpret disparity errors as bit errors.
For all cases, a pilot tone with a constant modulation index of 10% was generated over a range of 0.1 to 1.1 MHz.
For completeness also a reference measurement without a pilot tone was done. As seen from Fig 3a. the bias
modulation shows the penalty mentioned above. The performance of the VOA method shows an even higher penalty
than the bias modulation method, likely due to the non-linear tone distortions, which led to higher harmonics of the
tone signal modulation onto the high-speed data. As expected the FPGA variant shows a similar performance as the
case without modulation. We also measured the suppression of the harmonics, which were generated by the different
methods, in order to quantify the error in the presence of multiple tones. In general it was observed that the laser bias
and the VOA methods show a constant suppression ratio over all tone frequencies. For both variants the second
harmonic was the most significant with a suppression of ~50 dB (bias) and ~28 dB (VOA). However in the FPGA
JW2A.72.pdf
OFC/NFOEC Technical Digest © 2013 OSA
case the second harmonic was very low (suppression ratio >40 dB), while the third harmonic became the most
significant one. As shown in Fig. 3b, the suppression ratio of the third harmonic changes with frequency and is only
12.6 dB for a tone frequency of 1.1 MHz. However, as only tone frequencies up to 1.1 MHz are considered, third
harmonics of tone frequencies over 366 kHz would not fall into the tone frequency spectral range.
Fig. 3a. Experimental results
Fig. 3b. Third harmonic (3f0) suppression ratio of the FPGA method
5. Conclusion
We introduced a new method of generating a pilot tone using a modified 8b/10b line coding, which is compatible to
standard SFP transceivers. It was demonstrated that the new method is performing without any penalty compared to
standard 8b/10b encoding and, unlike conventional pilot tone generation schemes, does not incur degradation of the
high speed data signal.
6. Acknowledgement
The research leading to these results has received funding from the European Community’s Seventh Framework
Program (FP7/2007-2013) under grant agreement n° 249025 (ICT-OASE) and from the German ministry for
education and research (BMBF) under Grant 13N10864.
7. References
[1] H. Ji, K. Park, J. Lee, H. Chung, E. Son, K. Han, S. Jun, and Y. Chung, "Optical performance monitoring techniques based
on pilot tones for WDM network applications," J. Opt. Netw. 3, 510-533 (2004).
[2] Jin-Serk Baik, Kun-Youl Park, Tae-Won Oh, Chang-Hee Lee, "Analysis of penalty due to low-frequency intensity
modulation in optical transmission systems," Lightwave Technology, Journal of , vol.21, no.12, pp. 3300- 3307, Dec. 2003
[3] ANSI X3.230-1994, “Information Technology – Fibre Channel Physical and Signaling Interface. (FC-PH), with amendment
1, AM1-1996”
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