We demonstrate the transmission of 25Gb/s multimode optical signals over a record length of 300m multimode fiber designed for high modal bandwidth at 1310nm. The power penalty is 1.8 dB at 10−12 bit error rate level.
© 2013 Optical Society of America
There has been an increasing interest in using multimode fibers (MMFs) for high line rate short distance data communications in data centers and enterprise networks in recent years. A new standard is being drafted for 4x25 Gb/s 850nm VCSEL transmission over 100m OM4 MMF by the IEEE 802.3bm task force. The 100m maximum reach being considered is significantly shorter than the 550m reach specified for 10 Gb/s transmission. Restricted launches into a MMF using a single mode or quasi-single-mode VCSEL sources [1–4] have been suggested for overcoming the 100m length limitation encountered at 850nm. Some of the VCSELs described in these references have narrower line widths than multimode VCSELs, which can result in reduced chromatic dispersion effects; however the reliability of these devices has been questioned . Although using SMF is an obvious way to extend the system reach, the small fiber core size and the required high alignment precision result in high packaging costs for laser to fiber coupling, leading to higher optical transceiver costs.
In the current work, we propose using a MMF optimized for high bandwidth at 1310nm in conjunction with long wavelength sources such as integrated silicon-photonics (SiPh) transceivers  or VCSELs . This system retains the advantage of low loss coupling and passive alignment of conventional 850nm MMF systems. At the same time, the chromatic dispersion and attenuation of the fiber are much lower at 1310nm. This approach is different from experiments in which a long wavelength light source was launched into only the fundamental mode or into a limited number of modes of the MMF with the alignment tolerance similar to single mode systems [7, 8]. Therefore, it results in no advantage in coupling or alignment precision. Another experiment was reported on 40 Gb/s transmission over a 400m MMF using a 1300nm externally modulated external cavity laser as the light source under a restricted launch condition . While the eye diagram comparison was made with the help of an InGaAs PIN-photodiode, no quantified system performance assessment by bit error rate (BER) testing was performed using a proper receiver. The lateral alignment tolerance was only ± 2 μm, which is much more stringent than typical MMF systems.
In this paper, we demonstrate 25 Gb/s over a record 300m length of MMF using a true multimode launch into a MMF that was optimized for high modal bandwidth in the 1310nm window. This reach is three times that of the 100m reach proposed in standards bodies for OM4 fibers at 850nm and addresses the critical need of data centers to increase their line rates without sacrificing system reach.
2. Detailed technical results
2.1 MMF optimized for the 1310nm wavelength window
The core of a typical MMF has a graded index or “alpha” profile,
For our 1310nm optimized MMF design, the optimum α of 2.0 is slightly lower than for a MMF optimized for 850nm. To achieve high bandwidth with a multimode launch, we discovered that it is essential to incorporate a trench in the cladding region to equalize the modal delays of the outer mode groups [10–12]. The trench also significantly decreases the bend sensitivity compared to conventional multimode fiber. In our optimized design shown in Fig. 1, the trench for bending insensitive MMF (Bi-MMF) is located approximately 1.5 μm away from the edge of the graded index core and has a relative refractive index of −0.4%. The refractive index profile for standard MMF is also shown. The maximum value of Δ of the graded index core is 1%, which yields a numerical aperture of 0.2, while the core radius a is 25μm and corresponds to a 50 μm core diameter.
A 1310nm-optimized MMF based on the above description was fabricated, and 300m was deployed onto a shipping spool.
2.2 Encircled flux of multimode launch condition
A commercially available mode converter for 50 μm MMF  was used to convert the output of a 1310nm single mode laser into a VCSEL-like launch condition, as defined in 10Gb/s IEEE Ethernet Standard (for 850nm sources). The measured encircled flux from the multimode mode conditioner is plotted in Fig. 2 using open triangles. The calculated encircled flux from a single mode fiber operating at 1310nm is also shown, which illustrates that the output from the single mode fiber is very restrictive. Note that for 10Gb/s Ethernet Standard, the specified encircled flux launch condition requires that the integrated optical power be less than 30% at 9 μm diameter and greater than 86% at 38 μm diameter . These boundaries are marked by two solid dots in Fig. 1 and indicate that the launch condition used in our experiment is similar to the metric used to define 850nm VCSELs. Figure 2 also shows that there is a significant amount of power launched into the outer region of the 50 μm core, so achieving long reach with this condition is a much more stringent test of the modal bandwidth capability of the multimode fiber than using a single mode or restricted mode launch.
2.3 Modal bandwidth and Differential Mode Delay (DMD) of the MMF
We measured the transfer function of the MMF under the multimode launch condition using a frequency sweeping method  at 1310nm, yielding the results shown in Fig. 3. The Y-axis is in units of dBe, i.e. in the electrical domain as defined by 20Log() operator and is equal to 0.5dBo in optical domain as defined by 10Log() operator. The bandwidth for this fiber measured at the 6dBe or 3dBo level is 17.5 GHz, which translates to a bandwidth-length product of 5.25 GHz.km. We would note here that for 850nm optimized MMFs, the overfill bandwidth is specified to be at least 500MHz km with the majority falling between 500 and 700 MHz.km at 1310 nm. Hence, the system reach using those 850nm optimized MMFs is expected to be limited to tens of meters for 25Gb/s signals at 1310 nm as confirmed from our testing.
We also measured the DMD of a 1km length of the multimode fiber used in the experiments. That measurement is shown in Fig. 4 and indicates that the delays of the fiber are nearly the same for modes propagating near the center, middle and outer regions of the core. This means that bandwidth response of the fiber should be fairly uniform regardless of where the signal is launched into the course or whether connector misalignments or lenses transfer power between different modes.
2.4 Configuration of system testing
We evaluated the performance of the 1310nm-optimized MMF in a 25Gb/s transmission experiment using the testing setup shown in Fig. 5. The transmitter is based on a narrow linewidth 1310nm CW laser with a polarization maintained (PM) output through a PM fiber. The CW light is modulated by an intensity modulator from Photline operating at 25Gb/s at 1310nm, which provides the optically modulated signals through an single mode fiber. The mode conditioner then converts the single mode launch condition into a VCSEL-like multimode launch condition, as described in section 2.2. The optical receiver is a SiPh based multimode optical receiver provided by Intel . For BER testing, we utilized an Agilent BERT system operating at 25Gb/s. The controller (N4960A-CJ1) controls the pattern generator (N4951A-H32) and error detector (N4952A-E32). The controller also provides a clock signal to the pattern generator, which drives the modulator through a driver amplifier with properly set DC bias.
2.5 Eye diagrams results of bit error rate testing
Before doing the BER testing, we conducted detailed characterization of the transmitter and the transmitted signal as output from the fiber under test. An Agilent digital communication analyzer mainframe (86100D) with a multimode optical receiver plugin (86105D) and a precision time base (86107A) was used to condition the time signal provided by the controller. We used this setup to characterize various aspects of the eye diagrams including the signal rise time and extinction ratio.
We first measured the eye diagrams for both the back–to-back (B2B) condition and with the 300m length MMF inserted. Figure 6 illustrates that there is very little degradation of the optical eye after 300m. Table 1 provides the measured signal rise time and extinction ratio that were extracted from the eye diagrams. In the B2B condition, the eye diagram illustrates a 20/80% signal rise time of only 14.3ps, which is better than that being considered for the 4x25G standard by IEEE 802.3bm committee. We observed some degradation of the signal rise time and the extinction ratio when 300m of MMF was inserted into the system. The signal rise/fall time degrades from 14.3ps to 19.1ps as a result of fiber bandwidth limitation, and at the same time, the signal extinction ratio was also degraded from 14.26 dB to 9.45 dB. Nevertheless, with 300m MMF in place, the eye remained open and clean.
BER testing was conducted using a 231-1 PRBS data pattern. We first measured the BER vs. received power curve (referred as the waterfall curve) in the B2B configuration with the modulator output connected directly to the optical receiver. The received optical power was altered by changing the CW laser output power so that no additional variable optical attenuator (VOA) was needed. As shown in Fig. 7, in this B2B configuration, the receiver sensitivity was −13.6dBm average power. We then measured the BER vs. received power curve with 300m of MMF and the multimode launch condition shown in Fig. 2. As shown by the curve with circles in Fig. 6, the 300m length of MMF introduced a small level of impairment, with an additional 1.8 dB of optical power needed to bring the system performance back to around 10−12 BER. In further testing, the system was able to perform error free over a period of over 45 minutes when the average received optical power was set to −5.0dBm. Note the BER testing was conducted without a recovered clock for received signals.
We performed 25Gb/s system testing using a bend-insensitive MMF optimized for high modal bandwidth operation in the 1310nm window. The testing was conducted using a true multimode launch condition similar to the output from an 850nm VCSEL. We successfully demonstrated 300m transmission with a very low power penalty of 1.8 dB at 10−12 BER. The results illustrate that a carefully designed MMF is capable of long system reach at high data rates and is compatible with optical sources operating in the 1310nm window while maintaining the ease of connectivity and laser-fiber coupling. Encouraged by the work here, we have made further efforts to optimize the modal bandwidth of the MMFs, and to better understand the system performance of the full set of the Si-Ph transceiver. As a result, we were able to further extend the reach .
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