1. Introduction

Multimode fiber (MMF) is widely adopted in short-reach systems, such as local area network (LAN) and data center backbones, due to its ease of handling and high performance over short span. For example, high-speed networking standards like Fiber Channel, 10GbE, and 40GbE/100GbE have MMF links as a transmission medium. However, even in laser-optimized MMF, nominal transmission speed per wavelength does not exceed 10 Gb/s and transmission reach is shorter than several hundred meters. This is due to the difference of group velocities of various excited modes induces pulse distortion.

As the bandwidth demand of local area networks and data center backbones continues to rise, further increase of data rate per wavelength faster than 10 Gb/s has to be considered. For example, some industry experts believe that a standard describing 1 Tb/s should be available in the time frame of 2012–2013 [1]. Recently, there have been extensive studies to increase transmission capacity in MMF link. These include orthogonal frequency division multiplexing (OFDM) [2–4] and sub-carrier multiplexing (SCM) [6]. The OFDM technique splits high data-rate stream into many lower-rate sub-streams [2–5]. Its current record is 24 Gb/s per wavelength transmission speed with multimode vertical cavity surface emitting laser (VCSEL) [2]. The SCM technique utilizes very similar concept with OFDM, and 5.1 Gb/s transmission speed was reported using a single subcarrier [6]. One of other approaches depends on transmitter-based adaptive optics which adjusts launched field pattern to minimize modal dispersion [7]. The spatial light modulator used in adaptive optics approach induces most of energy launched into MMF has LP₀₁ mode. The idea behind this approach is to restrict the number of mode propagating through MMF to a few lower-order modes [8–9].

Previously, it was demonstrated that standard connector in single mode fiber (SMF)-pigtailed transmitter could restrict the number of modes launched into MMF (i.e. center launching technique) [10]. This is because the mode field pattern of SMF matches relatively well with the fundamental mode of MMF. The optimum condition of SMF to MMF launching was analyzed in [11]. According to the analysis, 98 % of optical power in the SMF can be coupled into the fundamental mode of MMF by simply fusion splicing SMF pigtailed transmitter with MMF link. The unwanted higher-order mode components generated by lateral offset between connectors within the MMF link or mode-mixing due to bending can be removed by mode filtering at the receiver side [9]. Thus, up to 40 Gb/s data-rate transmissions per wavelength over MMF were demonstrated with center-launching technique [11–14]. By combing with center-launching technique with wavelength-division-multiplexing, transmission capacity in MMF link could be upgraded easily [15]. In addition, because SMF occupies ~ 15 % of installed fiber at in-building networks [16], SMF transceiver approach could accommodate both MMF link as well as SMF link, simultaneously

In this paper, we experimentally demonstrate the feasibility of high speed signal transmission up to 86 Gb/s per wavelength through multimode fiber. We demonstrate transmission of 6 wavelength division multiplexed (WDM) × 86 Gb/s differential-quadrature-phase-shift-keying (DQPSK) signals over 2 km of OM2 grade multimode fiber with center-launching technique and multi-level modulation. The successful transmission of 86 Gb/s over multimode fiber is the highest speed per wavelength ever reported to our knowledge. The transmission performance over OM2 grade multimode fiber is also compared with OM3 grade multimode fiber and conventional single mode fiber.

2. Experimental setup

 

Fig. 1. Experimental setup for the feasibility demonstration of 86 Gb/s × 6 WDM channels over 2 km of MMF link (DFB-LD: distributed feedback-laser diode, AWG: arrayed waveguide grating, SM: single mode, MM: Multimode, EDFA: Erbium doped fiber amplifier, DR: data recovery circuit, CR: clock recovery circuit)

Download Full Size | PDF

Figure 1 shows experimental setup for the demonstration of 86 Gb/s × 6 WDM transmission over MMF. Transmitter was composed of continuous wave (CW) sources and external LiNbO3 Mach-Zehnder modulators. Distributed feedback laser-diodes (DFB-LD) operating from 1531.12 nm ~ 1542.14 nm with 200 GHz spacing were used as a light source. The output of these lasers were multiplexed, and then modulated with DQPSK format. The modulator produces four different optical phase of DQPSK signal [0, π/2, π, 3π/2] using integrated two-parallel MZ modulators with a relative optical phase of π/2. The parallel modulators were driven by two inverted/non-inverted 43 Gb/s pseudo-random binary sequence (PRBS) of non-return-to-zero (NRZ) electrical data (pattern length: 2⁷–1) signals. They were adjusted to have relative delay of 22-bit with each other before being applied into the modulator in order to de-correlate the data pattern. Thus, the aggregate bit rate of DQPSK signal was 86 Gb/s, while the symbol rate was 43 Gbaud. Total output power at the transmitter was - 3.5 dBm. The transmission link was utilized 2 km of OM2, OM3 grade MMF, or conventional SMF. The core diameter of both MMF was 50 μm. Overfilled launch (OFL) bandwidths of OM2 and OM3 graded fiber were 500 MHz.km and 1500 MHz.km at 850 nm, respectively. Therefore, it is worth to note that, if we use multimode VCSEL as a transmitter, the maximum reach of 10 Gb/s signal could be less than several hundred meters. The single mode (SM) to multimode (MM) launch at the transmitter side allows only limited number of modes excited into MMF link. When we launched the light into the MMF through a conventional SMF, more than 80 % of optical power in the SMF is coupled into the fundamental mode of the MMF. The coupling efficiency could be further increased by enlarging the mode-field diameter of SMF through fusion splicing [17]. In this experiment, we used fusion spliced SMF to MMF connectors. The MMF length of 2 km would be enough to evaluate the performance of high speed signal transmission over MMF because the length of installed MMF even at campus backbone is mostly less than 2000 m [16]. To compare the performance of MMF with that of SMF, we utilized conventional SMF (dispersion: 17 ps/nm/km @ 1550 nm) as a transmission link. The measured loss of MMF and SMF link were 2.4 dB and 1.2 dB, respectively. The MM to SM coupling at the receiver side provides additional mode filtering. In the case of MMF link, the received power variation was measured to be less than 0.5 dB. We used Erbium-doped fiber amplifier (EDFA) to supply enough power to high speed photo-detector as well as to change optical-to-signal-ratio (OSNR) sensitivity. For DQPSK signal reception, the differential optical phase between two arms of a single MZ interferometer with 1 bit delay was adjusted to ±π/4 to recover the in-phase and quadrature components of the DQPSK signal. Since the received bit stream was not a PRBS due to the nature of the DQPSK modulation, we programmed 43 Gb/s error detector with expected bit patterns to measure bit error rate (BER).

3. Results and discussions

Figure 2 shows the performances of both MMF and SMF links. To compare transmission performance in MMF link with SMF link, at first, we turned on only one channel operating at 1531.12 nm and turned off all other five channels. In this case, we measured BER curves by changing the loss of optical attenuator located at the two-stage EDFA, which induced OSNR variation of received signal. We set the output power of EDFA to 12 dBm regardless of OSNR. Figure 2(a) represents OSNR sensitivities after transmission of 86 Gb/s DQPSK signal over 2 km of MMF and SMF. The back-to-back receiver sensitivity was 26.4 dB at the BER of 10^-9 . After 2 km transmission over SMF link, OSNR sensitivity was degraded ~ 4.4 dB due to chromatic dispersion of transmission link as well as clock recovery circuit. Because the clock recovery circuit used in this experiment was optimized for 43 Gb/s NRZ signal, the recovered clock was not clear enough when chromatic dispersion was involved. However, there was small OSNR sensitivity difference between OM3 grade MMF and SMF, which confirms the possibility of 86 Gb/s transmission speed per wavelength over MMF link. The OSNR sensitivity difference between OM3 grade fiber and OM2 grade fiber was less than 0.2 dB. There was no sensitivity difference between in-phase and quadrature components of the DQPSK signal. Figure 2(b) represent received eye-diagrams measured before and after delay interferometer. Measured eye-diagrams after 2 km transmission show little distortion due to chromatic dispersion compared with back-to-back configuration. We have clear eye-opening with delay interferometric detection for OM2, OM3 grade MMF, and SMF links.

We turned on all six channels at the transmitter and measured the OSNR sensitivities, as shown in Fig. 3. In this case, we used OM2 grade MMF as a transmission medium. The power of received signals showed no significant loss variation among channels, as shown in Fig. 3(a). Figure 3(b) shows the measured OSNR sensitivities for WDM channels. 6 × 86 Gb/s WDM transmission over 2 km of MMF was achieved without error-floor. The sensitivity variation among channels measured at the BER of 10–9 was less than 1 dB. Thus, we achieved a bit-rate-distance product of 1.032 Tb/s-km (6 × 86 Gb/s, 2 km) capacity over OM2 grade multimode fiber. The successful transmission of 86 Gb/s over multimode fiber is the highest speed per wavelength ever reported to our knowledge. Further capacity increase with WDM technique is straightforward. We expect that total transmission capacity over MMF could be easily increased many fold by introducing more WDM channels.

 

Fig. 2. Measured BER curves and eye-diagrams in case of a single channel transmission (a) OSNR vs. BER curves (b) eye-diagrams

Download Full Size | PDF

 

Fig. 3. Measured optical spectra and BER curves for WDM transmission when OM2 grade fiber was utilized (a) optical spectra (b) OSNR vs. BER curves

Download Full Size | PDF

4. Summary

We have experimentally demonstrated the feasibility of high speed signal transmission up to 86 Gb/s per wavelength through multimode fiber. We demonstrated 6 × 86 Gb/s differential-quadrature-phase-shift-keying (DQPSK) signals over 2 km of OM2 grade multimode fiber with center-launching technique and multi-level modulation. The OSNR sensitivity variation among WDM channels measured at the BER of 10^-9 was less than 1 dB. The successful transmission of 86 Gb/s over multimode fiber is the highest speed per wavelength ever reported to our knowledge. The transmission performance over OM2 grade multimode fiber was also compared with OM3 grade multimode fiber and conventional single mode fiber. There was no significant performance dependence on the media. From these results, we expect that 100 Gb/s per single wavelength transmission over multimode fiber will be also possible.