10-Gb/s Pulse-Amplitude ModulatedTransmission Over 1-mm Large-Core
Polymer Optical FiberSven Loquai, Roman Kruglov, Christian-Alexander Bunge, Member, IEEE,
Olaf Ziemann, and Bernhard Schmauss, Member, IEEE
Abstract— The authors report on a 10-Gb/s transmission over1-mm large-core diameter polymer optical fiber (POF) usingpulse-amplitude modulation. For the first time, a real-time10-Gb/s eye-diagram is shown after 5-m standard step-indexPOF (SI-POF), even with an eye-safe transmitter (0 dBm).With an offline decision feedback equalization technique, a linklength of 10-m SI-POF, respectively, 25-m GI-POF was achieved.With a slightly higher optical power of +5 dBm, the maximumlink length could even be increased to 30-m SI-POF and 60-mGI-POF.
Index Terms— Eye-safe, optical communication, polymeroptical fiber, pulse-amplitude modulation.
I. INTRODUCTION
POLYMER optical fiber (POF) is a promising transmissionmedia for broadband in-home networks but also for inter-
connection and short-reach connections up to 100 m, wherelow complexity transceivers and low latency are importantissues.
With advanced modulation techniques like discrete multi-tone modulation (DMT), the potential of large-core 1 mm POFto transmit 10 Gb/s was already demonstrated [1-3].
But while this DMT modulation format makes good useof the available bandwidth, it requires considerable signalprocessing at the transmitter and receiver. It also adds latencyto the transmission due to the block-wise nature of the DMTmodulation format.
In the given letter we demonstrate for the first time pulse-amplitude modulation (PAM-4) of 10 Gb/s over 1 mm large-core POF using a newly developed optical POF receiver com-prising a large area photodetector (PD) with a transimpedanceamplifier (TIA).
Manuscript received January 9, 2012; revised February 16, 2012; acceptedFebruary 19, 2012. Date of publication February 27, 2012; date of currentversion April 18, 2012. This work was supported in part by the FederalMinistry of Education and Research (BMBF) under Project 17016X10.
S. Loquai, R. Kruglov, and O. Ziemann are with the Polymer Optical FiberApplication Center (POF-AC), University of Applied Sciences Nuremberg,Nuremberg 90489, Germany (e-mail: [email protected];[email protected]; [email protected]).
C.-A. Bunge is with the Hochschule f. Telekommunikation, DeutscheTelekom AG, Leipzig 04277, Germany (e-mail: bunge@hft leipzig.de).
B. Schmauss is with the University of Erlangen-Nuremberg, Erlangen91058, Germany (e-mail: [email protected]).
Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2012.2189002
Fig. 1. Schematic transmission setup used for the experiments.
With this modulation format both issues of DMTtransmission can be addressed. In contrast to DMT a muchlower complexity is needed to modulate and demodulate thesignal which results in a lower latency, lower power consump-tion and lower cost. Therefore PAM is a promising candidatefor next-generation, low-latency 10 Gb/s transmission overlarge core 1 mm polymethylmetacrylate (PMMA) POF.
II. TRANSMISSION SETUP
The transmission setup is shown in Fig. 1. The edge-emitting laser diode (LD) or the Vertical-Cavity Surface-Emitting Laser (VCSEL) is directly driven with a pseudorandom bit sequence mapped to a PAM-4 signal.
The PAM-4 signal is generated by an arbitrary waveformgenerator (AWG) with a resolution of 10 bit and a samplingrate of 10 GSa/s.
The transmitters used in the experiment are a commerciallyavailable edge-emitting laser diode with a wavelength of650 nm and an optical output power of 7 mW, respectively,a VCSEL (Firecomms) with a wavelength of 660 nm and anoptical output power of 1 mW which corresponds to eye-safeoperation. The LD and the VCSEL are driven in their linearregion with a modulation index of approximately m ≈ 0.8.
The new developed POF receiver has an active diameter of∅300 µm. A dielectric taper was used to increase the couplingefficiency between the POF and the photodiode.
The real time oscilloscope captures the analog electricalsignal with a resolution of 8 bits. For the offline processing
Fig. 2. Comparison of BER versus received power after 35-m GI-POF.
Fig. 3. Comparison of Gaussian distribution of the PAM-4 histogram.
a symbol-spaced DFE with 8 feedforward and 8 feedbacktaps was used. The weights of the filter are estimated bya recursive least-square (RLS) algorithm on the basis of atraining sequence. The bit-error ratio is then evaluated by errorcounting and Q-factor estimation.
III. MEASUREMENT RESULTS
Due to a limited memory of the oscilloscope (6 millionsymbols) an evaluation of a BER < 10−6 was not feasible witherror counting. Therefore several measurements were carriedout, to analyze whether the measured Q-factor can be usedfor an estimation of the BER according to [4]. Fig. 2 showsthe comparison between the BER obtained with error countingand with Q-factor estimation versus the received optical powerafter 35 m GI-POF. The Gaussian distribution around eachPAM level in Fig. 3 of this 35 m PAM-4 FFE/DFE experimentcorresponds to a received optical power of −6.6 dBm.
It can be seen from Fig. 2 that direct error counting andQ-factor estimation of the measured system are in goodagreement and the histogram of the received PAM-4 signalin Fig. 3 has a Gaussian distribution.
A. Real-Time Results With Passive Analog Equalization
In an approach to see if an adaptive analog filter with alower complexity and a lower power consumption comparedto a FFE/DFE equalizer can be used, a simple analog filter wasdesigned to equalize the signal and to reduce the influence ofinter-symbol interference (ISI).
With this setup it was possible to transmit the signal in real-time over 5 m of 1 mm SI-POF using the eye-safe VCSEL.
Fig. 4. Real-time equalized eye-diagram after 5-m SI-POF using an eye-safetransmitter.
Fig. 5. Frequency response of the system (5-m SI-POF including a 25-mmbend near the transmitter) and the equalizer.
Fig. 4 shows the real-time transmitted PAM-4 eye-diagramover 5 m SI-POF. The evaluation of the Q-factor indicates aBER of 10−6.
Due to the small numerical aperture (NA) of the usedVCSEL no equilibrium modal power distribution is achievein the fiber for short distances. Hence, the dispersion char-acteristics in the fiber changes when the fiber is subject totight bends (< 25 mm) especially near the transmitter. Bendsnear the receiver or bends larger than the specified minimumbend radius of 25 mm have almost no effect on the frequencyresponse and just result in an additional attenuation. For bendsbelow 25 mm multi-core (MC) POF may be used [5].
The frequency responses of the POF system (includingtransmitter, SI-POF and receiver), of the analog filter, of theequalized POF system and the influence to a fiber bend (360°)are shown in Fig. 5.
B. Measurement Results With FFE/DFE
To further increase the link length an offline FFE/DFEequalizer was applied. The BER was then obtained with error
LOQUAI et al.: PULSE-AMPLITUDE MODULATED TRANSMISSION OVER 1-mm LARGE-CORE POF 853
Fig. 6. Computed 10 Gb/s eye-diagram after 60-m GI-POF (BER<10−3).
Fig. 7. Histogram for 35-m (BER < 10−9), 50-m (BER < 10−6), and 60-m(BER < 10−3) GI-POF.
TABLE I
MEASUREMENT RESULTS FOR 10 Gb/s PAM-4 OVER LARGE-CORE 1-mm
PMMA POF USING THE EYE-SAFE VCSEL
System setup BER(error counting)
BER(Q-factor estimation)
5 m SI-POF no errors counted 8.9•10−10
10 m SI-POF no errors counted 1•10−7
10 m GI-POF no errors counted 4•10−10
25 m GI-POF 1•10−4 1.6•10−4
counting and with Q-factor estimation. With the use of theeye-safe transmitter and the offline FFE/DFE equalizer an errorfree transmission was achieved for a link length of 5 m of1 mm SI-POF and 10 m of 1 mm GI-POF. An evaluation ofthe Q-factor indicates that the received signal has a BER below10−9. If a Reed-Solomon forward error correction (FEC)with 7% overhead is used, the link length can be increasedto 10 m SI-POF and 25 m GI-POF assuming a FEC limitof 10−3 [6].
In addition, the red (650 nm) laser diode with a fibercoupled power of +5 dBm was used to show the potential
TABLE II
MEASUREMENT RESULTS FOR 10 Gb/s PAM-4 USING THE
EDGE-EMITTING LD WITH A FIBER-COUPLED POWER OF +5 dBm
System setup BER(error counting)
BER(Q-factor estimation)
10 m SI-POF no errors counted 1•10−10
30 m SI-POF 9•10−4 8.2•10−4
35 m GI-POF no errors counted 1•10−9
60 m GI-POF 7•10−4 4.5•10−4
of this robust, low latency transmission POF-system forthe use in active cables. A link length of 35 m of 1 mm GI-POFwith a BER below 10−9 and even 60 m with a BER < 10−3
was achieved. Fig 6 shows the computed eye-diagrams after60 m of 1 mm GI-POF which corresponds to a BER < 10−3.The evaluated histograms for 35 m, 50 m and 60 m of 1 mmGI-POF are shown in Fig. 7.
Tables 1 and 2 summarizes the measurement results thatwere obtained with the eye-safe VCSEL (0 dBm) and theedge-emitting LD (+5 dBm), respectively.
IV. CONCLUSION
With an optimized system, including a linear receiver forlarge-core polymer optical fibers, 10 Gb/s were transmittedover 10 m (eye-safe VCSEL) and over 35 m (+5 dBm edge-emitting LD) of 1 mm large-core PMMA-GI POF with aBER < 10−9.
An error free (BER < 10−9) transmission was also achievedover 5 m of 1 mm step-index POF using the eye-safe VCSEL.The simple PAM-4 modulation scheme, without the need forerror correction results in much lower latency and complexitycompared to DMT modulation.
If a FEC is used, the link length can be increased to 25 m of1 mm GI-POF or 10 m SI-POF using an eye-safe transmitter.For an optical output power of +5dBm a link length of60 m and of 30 m was achieved with GI-POF and SI-POF,respectively. In combination with the achieved real-time eye-diagram after 5 m SI-POF using an eye-safe transmitter, largecore 1 mm POF is a powerful competitor to current high-speedcopper solutions.
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