Multi-gigabit transmission over 1 mm core diameter graded
index POF
Citation for published version (APA):
Okonkwo, C. M., Tangdiongga, E., & Koonen, A. M. J. (2011). Multi-gigabit transmission over 1 mm core
diameter graded index POF. In O. Ziemann (Ed.), POF-PLUS handbook : handbook of the European PDF-PLUS project 2008-2011 (pp. 85-89). PDF Plus project.
Document status and date: Published: 01/01/2011 Document Version:
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Multi-Gigabit Transmission over 1 mm Core
Diameter Graded-Index POF
Chigo Okonkwo, Yan Shi, Eduward Tangdiongga and Ton Koonen COBRA Research Institute, Eindhoven, The Netherlands
Abstract: One of the main objectives of the EU POF-PLUS project is the
optimization of available components and techniques to achieve multi-gigabit transmission over 1 mm core diameter plastic optical fiber. Hence the work in this section, demonstrates transmission rates between 5.3 and 7.6 Gbit/s achieved for fiber lengths between 10 and 50 m using discrete multitone modulation (DMT) in an intensity-modulated direct-detection system using directly modulated eyesafe VCSEL and silicon photodiode (PD). The used system bandwidth is only 1.42 GHz corresponding to a spectral efficiency of >3.7 bits/s/Hz.
1. Introduction
In order to achieve the ambitious goal of multi-gigabit transmissions over plastic optical fibre, careful optimization of key factors is vital: the optimi-zation of the optoelectronics to maximize bandwidth and linearity of low-cost transceivers, the selection of the most efficient modulation format such as On-Off Keying (OOK) or spectral/power efficient advanced modulation formats such as pulse amplitude modulation and discrete multi-tone (DMT) [1]. Finally, the selection of the POF type amongst the different solutions commercially available is vital. Whilst, conventional standard A4a.2 step-index (SI) POF has been shown for gigabit transmission however, the numerical aperture (NA) of 0.5 presents a low bandwidthdistance product (80 MHz at 50 m) [2]. Therefore, for multi-gigabit transmission (over lengths suitable for in-home deployments), the other option available is to use graded-index (GI) POFs.
2. Experimental Setup and Discussion
Employing the experimental setup shown in Fig. 1, the low-cost VCSEL at 667 nm with a bandwidth of 3 GHz was directly modulated by the output of the arbitrary waveform generator (AWG) running at a sampling rate of 4.5 GHz. The bias current of the VCSEL was set to 3 mA, hence a peak-to-peak driving current of maximum 6 mA.
Fig. 1: Experimental Setup
Corresponding to the levels set for ANSI eye safety regulations, an optical power of 0 dBm was launched into the POF link. After signal transmission over 50 meters single-core GI-POF link, the optical power received was -15 dBm mainly attributed to attenuation in the POF (0.3 dB/m at 667 nm). In this work, a low-cost Silicon photodetector with a photo sensitivity area of diameter 400 μm and a responsivity of 0.5 A/W in the red light region is used to receive the signals. In addition, it is equipped with a trans-impedance amplifier (TIA) with a trans-impedance gain of 10 kΩ. The receiver is optimized for DMT with a bandwidth of more than 1.6 GHz. The maximum available bandwidth after 50 m is approximately 1.3 GHz as shown in Fig 2a. By using the adaptive bit-loading algorithm, the limited bandwidth can be used efficiently, hence increasing the spectral efficiency. The DMT (de-) modulation is executed offline to calculate the optimal
bit-loading parameters. Using a real-time oscilloscope running at a sampling rate of 50 GSamples/s, the received signal is evaluated by off-line processing in Matlab®. The high sampling rate is due to lack of synchronization between the AWG and real-time scope; hence phase information has to be extracted from the DMT frame. However, with suitable cyclic prefix consisting of synchronisation bits, it is possible to reduce the sampling rate to currently available FPGA speeds. In this experiment, we chose 256 subcarriers, ranging from 0 to 2.25 GHz. The choice of this number of subcarriers is ultimately a trade-off between processing requirements and the achievable transmission rate.
Fig. 2: (a) Normalized frequency response of the system for back-to-back and 50 m GI-POF configurations (b) Transmission performance for 50 m: (upper) bit, allocation and (lower) SNR
The aim is to maintain a low number of subcarriers and few bits per subcarrier, hence less processing is required, making the implementation of a low-cost device possible. The algorithm measured the SNR per subcarrier, which is shown as a function of frequencies in Fig. 2a. Accordingly, the algorithm allocated bits per subcarrier to obtain an optimum transmission rate. Notice that the received SNR values present a stair-like curve shown in Fig 2b (lower). Also, shown is a truncation at the frequency of 1.42 GHz above which no bits are assigned. In Figure 2b (upper), 5 bits are allocated mainly between the 21st and 45th subcarrier group reducing to 0 bits being assigned beyond the 162nd subcarrier or 1.42 GHz. This truncation is largely caused by the system bandwidth. Moreover, the robustness of the system is further demonstrated in Figure 2b (upper) where the power-loading algorithm efficiently allocates bits to the carriers corresponding to the available SNR determined by the response of the system.
In Figure 3a, the measured values of BER after 50 m POF transmission for all the subcarriers is shown. Notice that most subcarriers meet the target BER of 10-3, which is chosen based on the forward error correction (FEC) limit for error-free operation. Reducing the POF length results in the following transmission rates; for BER below 10-3, transmission rates of approximately 6.2, 7.2 and 7.6 Gbit/s are achieved over 35, 20 and 10 m respectively.
Fig. 3: (a) BER for different subcarriers at 5.3Gbit/s over 50 m. (b) Constellation of received signal 32 QAM for highest subcarrier indices.
For the target POF length of 50 m, we are still able to achieve 5.3 Gbit/s which is a record transmission rate. All bit rates mentioned in this paper includes the 7% FEC bits, cyclic prefix and preambles.
Conclusion
By employing eyesafe offtheshelf transceivers for transmission over 1 mm core diameter PMMA GIPOF with, a costeffective solution is presented for realizing multigigabit transmission. This solution presents an ideal doityourself installation in comparison to coaxial and twisted pairs solutions as it can be installed in the same powerline ducts. In combination with emerging realtime digital signal processing, scalability towards 10 Gbit/s shortrange communication over 1 mm core diameter POFs is feasible.
References:
[1] S.Randel, F. Breyer, S.C.J. Lee and J.W. Walewski, “Advanced modulation schemes for short-range optical communications”, JSTQE, vol. 16, no. 5, p. 1280-1289, 2010
[2] S. Randel, S.C.J. Lee, B. Spinnler, F. Breyer, H. Rohde, J.Walewski, A.M.J. Koonen, and A. Kirstädter, “1 Gb/s transmission with 6.3 bit/s/Hz spectral efficiency in a 100m standard 1mm step-index plastic optical fibre link using adaptive multiple sub-carrier modulation”, Proc. ECOC 2006, paper Th4.4.1
[3] D. Visani, C. M. Okonkwo, S. Loquai, H. Yang, Y. Shi, H. P. van den Boom, T. Ditewig, G. Tartarini, B. Schmauss, S. Randel, T. Koonen, and E. Tangdiongga “Record 5.3Gb/s transmission over 50 m 1 mm core diameter graded-index plastic optical fiber”, Proc. OFC 2010, paper PDPA3
