1. Introduction

Multimode fiber (MMF) links are currently attracting much interest as the transmission medium for gigabit per second local area networks (LANs). Such high-speed links are particularly needed for backbone links, because of the increased LAN bandwidths following the 10-Gigabit Ethernet (10GbE) standard. The majority of installed in-building fiber (approximately between an 85% and a 90%) consists of 62.5/125 µm silica multimode fiber, as recommended by the ISO/IEC 802.11 standard. There is recently an important drive to utilize this existing infrastructure to support the ever increasing capacity demand because MMFs bring multiple advantages regarding to their ease of installation, maintenance and handling, which in turn implies an important cost reduction. However, the transmission capacity of these in-building networks is severely limited by intermodal dispersion. Despite of this disadvantage, recent efforts have pointed that MMFs can be employed to support high-speed digital connections such as those required by GbE applications for transmission speeds up to 10 Gb/s in short-reach distances. Furthermore, it has been demonstrated that the use of laser instead of LED based transmitters results in a considerable improvement of the MMF link transmission bandwidth.

Different novel techniques are now being developed to achieve this target such as mode group diversity multiplexing Ref. [1–4], optical frequency multiplication Ref. [1,5], the application of MIMO (Multiple Input Multiple Output) techniques Ref. [6], and subcarrier multiplexing (SCM) Ref. [7–11,13]. Among these approaches, the last one become especially interesting since it provides the potential for the delivery of radio and wireless services and also for broadband digital transmission by suitable combination of the traditional SCM with Orthogonal Frequency-Division Multiplexing (OFDM), Ref. [12]. In addition, some recent results have demonstrated that the use of low-linewidth lasers can support the delivery of radio over fiber (ROF) signals beyond the bandwidth-distance product of typical installed MMF Ref. [13–15], for a range of microwave frequencies up to 20 GHz through MMF links up to 5 km.

Another important approach to force less intermodal dispersion consists in reducing the number of modes propagated through the fiber by means of practical mode-filtering implementations at the transmitter and/or receiver side, Ref. [16–18]. The transmitter side mode-filtering is performed by exciting a limited number of lower order modes in the MMF and thus coupling only a small portion of the total power into the rest of the transmitted modes. This will be implemented in this paper by central launching the light from a SMF, which in turn entails two notable implications: it concentrates most of the energy in the axial core region and reduces the amount of energy in the higher order modes thus reducing the effect of modal dispersion from the coupling of the higher order modes to the lower ones. The mode-filtering at the receiver side consist in recovering the lower order propagated modes which implies some signal energy loss. The reception at the reported 5 km transmission is implemented by coupling the MMF link end directly to a SMF. It must be taken into account that it would be possible to filter the lower order modes instead of the higher ones, but it has not been considered here because it will require more sophisticated launching techniques.

It is also possible to increase even more the total transmission capacity of MMF links by combining wavelength division multiplexing (WDM) with some of the above proposed techniques, Ref. [8,9] and Ref. [19,20].

In this paper we report the experimental demonstration of 10×20 Gb/s data transmission channels using 10 dense WDM channels over the 200-GHz ITU grid’s C-band through a 5 km 62.5-µm core-diameter graded-index multimode silica fiber link. Successful error-free transmission is achieved by employing low-linewidth Distributed FeedBack (DFB) lasers and performing the mode-filtering approach at the transmitter end by means of Singlemode Fiber (SMF) central launch.

2. Experimental system

The scheme of the setup for the experimental demonstration is shown in Fig. 1. The 20 Gb/s NRZ signal was modulated onto 200-GHz (1.6 nm) spaced wavelengths over the ITU grid on the C-band resulting in a throughput of 200 Gb/s. Each of the 10 CW signals provided by the low-linewidth DFB lasers, characterized by a linewidth range of 1–10 MHz, delivered 8 dBm optical output power, wavelengths ranging from 1540.56 to 1554.94 nm were combined using a commercially available 40-channel AWG (Arrayed Wavelength Grating). Prior to their modulation by the Parallel Bit Error Ratio Tester (ParBERT) electro-optic modulator, each channel polarization state was conveniently controlled by a polarization controller. Following their modulation with a 20 Gb/s 2³¹-1 pseudorandom bit sequence (PRBS) NRZ data stream, the 10 optical channels were central launched from the 9-µm core-diameter single-mode fiber into 5 km of 62.5-µm core-diameter graded-index (parabolic core grading, α=2) silica multimode fiber with an standard first order chromatic dispersion parameter of D=17 ps/(nm·km). The applied central launching scheme involved a radial offset ranging from 0 to 2 µm. The 5 km link propagation assured the properly time decorrelation of the PRBS sequence in adjacent channels which is in fact equivalent to having transmitted the channels previously modulated with different 2³¹-1 PRBS patterns. The SM-MM launch scheme provides the initial spatial mode filtering effect by launching only a limited number of lower order modes into the MMF link.

At the receiver end, the MMF end is coupled to a SMF fiber in order to filter the higher order modes propagated through the MMF, which were excited by intermodal coupling from the initially launched modes. It must be noted that this filtering approach removes some of the signal energy and thus results in some optical loss. Each of the 10 possible channels were wavelength selected using another 40-channel AWG, the output of which was amplified by an erbium-doped fiber amplifier (EDFA) providing about 20 dB gain before being properly detected and analyzed by the Communications Signal Analyzer (CSA) and the 40 Gb/s ParBERT analyzer.

 

Fig. 1. Block diagram of the experimental setup for WDM transmission

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Fig. 2. Measured frequency response of the 5 km multimode fiber link

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3. Results and discussion

The potential of MMFs for broadband ROF transmission in the microwave region has been theoretically justified in Ref. [21] as a consequence of the nonideal behavior as a microwave transversal photonic filter of a MMF link. It was also shown that this possibility of exploiting the MMF response at frequencies far from baseband is contingent on the use of low-linewidth lasers. In order to confirm the results obtained in Ref. [21] and previous to the 200 Gb/s digital transmission, we measured the electrical frequency response of the 5 km MMF link for radio frequencies up to 25 GHz. We employed the internal ParBERT DFB laser centered at 1550.12 nm and an external electro-optic (EO) intensity modulator with a 3 dB RF bandwidth of 15 GHz. After the 5km MMF link, the light was detected using an MMF-pigtailed 22-GHz-bandwidth Discovery Semiconductors DSC30S PIN photodiode.

Fig. 2 shows that the measured normalized response obtained with an Agilent E8364A Network Analyzer (45 MHz - 50 GHz) is in agreement with the model presented for radio over fiber systems, Ref. [21]. We clearly appreciate that the contrast ratio between the transversal filter resonances and the secondary side-lobes is dramatically reduced. This is due to the fact that the MMF link behaves as an imperfect transversal filter, i.e. the difference in the propagation delays between adjacent mode groups is not a constant value. In fact we can see that the measured response is relatively flat with maximum variations of ± 2 dB with respect to a mean level of approximately 2 dB below the low frequency regime. It must be noted that the fall produced above the 20 GHz is a consequence of both the external modulator and the detector 3-dB RF bandwidths. With all, we reach the conclusion that our 5 km MMF link is suitable for transmission of passband channels over a broad region of frequencies.

Although mode-filtering at the receiver end is required in the WDM experimental setup described in Fig. 1, due to the lack of a multimode demultiplexing device, we also performed multimode detection with the MMF-pigtailed PIN photodiode followed by a 40-GHz-bandwidth 30-dB-gain RF amplifier for independent transmission of every channel. This ensures that error-free transmission of a baseband channel extending into the radiofrequency region is also achievable even if no multimode to singlemode transition is present at the end of the link. To evaluate the performance of this setup, the recovered eye diagram and the corresponding quality factor Q for 20 Gb/s transmission are shown in Fig. 3 for the 1551.72 nm channel. It is thus demonstrated that the coupling of all the receiver modes to the detector area doesn’t degrade the potential of high-frequency broadband radio over fiber transmission.

 

Fig. 3. Received eye diagram and quality factor Q for multimode detection

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Finally, we mounted the setup with MM to SM conversion illustrated in Fig. 1, to perform the wavelength multiplexing/demultiplexing of the channels. Fig. 4 shows the recovered eye diagrams after simultaneous transmission over the 5-km MMF link for every 20 Gb/s received WDM channel ranging from 1540.56 nm to 1554.94 nm. It can be seen that open eye diagrams are observed for every selected wavelength resulting in a high measured quality factor Q that ranges from 8.13 to a value of 9.74, what implies a Bit Error Rate (BER) < 10^-15 for a received optical power of +1 dBm. Comparing these results what those obtained for multimode detection in Fig. 3, it can be observed a Q factor improvement which is attributed to the optical power gain provided by the EDFA located at the output end of the second AWG.

 

Fig. 4. Recovered eye diagrams and quality factors Q for every demultiplexed WDM channel

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Figure 5 shows the measured points for the BER for every one of the 10 received channels versus the optical receiver power. Despite the differences in the received optical power level between channels, due in part by the different attenuation suffered in the mux/demultiplexing process, one can affirm that all the wavelength selected channels follow a similar BER behavior. For all channels error free transmission is achieved with received powers in the -7 to -4.5 dBm range.

 

Fig. 5. Bit error rate versus optical received power for every WDM channel

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Table I shows the measured power penalty for every transmitted channel at the BER of 10^-9. The SM-MM launching scheme causes a power penalty of 2 dB, which is in line with the results presented in Ref. [18] for the transmission of 10 Gb/s signal over 3.7 km MMF link for mode-field matched center launching at the same BER.

Table 1. Power penalty for every WDM channel

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4. Summary and conclusions

We have reported two important results in this paper. On one side we have demonstrated that by combining central launching through a singlemode fiber and a low linewidth laser the transmission of broadband signals (from baseband to radio regions) is feasible in a 5 Km long MMF link even if multimode detection is employed. An achievable bandwidth of more that 10 GHz can be achieved which has been demonstrated by successfully transmitting a 20 Gb/s single channel.

Secondly a record value of 1 Tb/s·Km has been achieved in a multimode fiber link by combining the former capability with WDM transmission. In particular we have successfully transmitted 10×20 Gb/s 200 GHz-spaced ITU channels through a 5 km link of 62.5-µm core-diameter graded-index multimode silica fiber with an error rate better than 10^-15. This corresponds to an aggregate bit rate per length product of 1 Tb/s·km, the highest value ever reported to our knowledge for MMF links.