Study of long-wavelength directly modulated VCSEL transmission using SOA amplifiers Chrostowski, L. (1), Chang C. H., Stone, R. J. *, Chang-Hasnain, C. *
(1)
[email protected] EECS Department, U.C. Berkeley, California, USA R. J. Stone is at Bandwidth 9 Inc., Fremont, California, USA
Abstract: We demonstrate the viability of high-performance WDM VCSEL transmission with semiconductor optical amplifiers as a low cost alternative to systems composed of DFB lasers with EDFA amplification. We study the use of Corning MetroCorâ fiberä as a means of combating the dispersion effects resulting from the chirp of directly modulated lasers. Introduction Current high-performance communication systems are composed of distributed feedback (DFB) lasers with erbium doped fiber amplifiers (EDFA) throughout the links. Though the performance of these links is superb, a more cost effective solution exists for deployment in metro applications, as demonstrated by the VCSEL datacom market. For such shorter distance systems, VCSELs and semiconductor optical amplifiers (SOAs) are the likely candidates due to their low component costs and small package footprint. Short-wavelength VCSELs have been available for a decade, however, it is only recently that VCSELs at telecommunications wavelengths (1.55 µm) have been available /1/. Their applicability for 2.5 Gb/s WDM systems has also been demonstrated /2/. EDFAs typically have superior performance to SOAs. Because the SOA has faster gain dynamics than the EDFA, the SOA suffers from pulse distortion and crosstalk when operated under saturation. Also, SOAs have a higher noise figure than EDFAs. In this study, we demonstrate that even with these impediments, it is still possible to achieve high performance WDM transmission using VCSELs and SOAs. We compare Corning’s SMF-28 to MetroCorâ fiberä, and study the effects of dispersion. MetroCor fiber has negative dispersion while directly modulated sources have positive chirp; the interaction leads to pulse compression. /3/
wavelengths were 1590.25 and 1591.84 nm. The VCSELs were biased using an HP 4155 semiconductor parameter analyser. A bias-tee was used to combine the DC and RF from the bit error rate tester. The DC and RF amplitudes were optimised to provide the best bit error rate performance. Independent PRBS 223-1 2.5 Gb/s signals were applied to each laser. In-line optical fiber isolators were used to provide optical isolation. The multiplexing and demultiplexing of the signals was done using a Lightwave Microsystems WDM filter. The signals were either preamplified using a commercial JDSU Inc. SOA or a commercial EDFA. The signals were then transmitted through 50 km of MetroCor or SMF-28 fiber. Because of the high insertion loss of the mux and demux (8-10 dB each), the transmission distances were powerbudget limited to 50 km. Detection of the signals was performed using an Epitaxx Inc. avalanche photodiode (APD) with a sensitivity of –32 dBm.
Experiment The VCSEL devices used in the WDM system were comprised of n-doped InAlGaAs/InAlAs bottom DBRs, with an InGaAs multiple quantum well active region, lattice matched to the InP substrate. The p-type top mirror was metamorphic GaAs/AlGaAs. Current and optical confinement were provided by selective oxidation. /1/ Ion implantation was performed to improve the frequency response. /4/
Figure 1: System diagram Figure 1 shows a schematic diagram of the WDM system. Two VCSELs were used to transmit 2.5 Gb/s data on channels with a 200 GHz spacing. The channel
Figure 2: Experimental BER for 50 km MetroCor; comparing SOA with EDFA Figure 2 shows the bit error rate curves comparing VCSEL transmission using an SOA or EDFA as a pre-amplifier, both through 50 km of MetroCor fiber. The left-most curve is the back-to-back link, for reference; this is comprised of one VCSEL modulated at 2.5 Gb/s, coupled into an FC/APC connector, through an isolator, an attenuator, then directly to the avalanche photodetector. The middle curve corresponds to two-channel transmission through fiber using the EDFA, while the right-most curve is two-channel transmission using the SOA. The SOA shows a power-
penalty of 1.2 dB at a BER of 10-9 compared to the EDFA. No crosstalk between channels was observed (i.e. identical traces were observed with only one laser turned on). No BER floor was observed.
eye-diagram iterations performed (in order to reduce computation time). At the time of the conference, more simulations with a larger number of iterations will be discussed. The x-axis is in arbitrary units. Figure 4 also shows the simulated 2.5 Gb/s eye diagrams, for a received power corresponding to –27 dBm on the graph’s scale. Dispersive effects are clearly visible for the SMF-28 fiber.
Figure 3: Experimental BER curves using EDFA; SMF-28 vs. MetroCor fiber. Figure 3 shows the bit error rate curves comparing VCSEL transmission through SMF-28 and MetroCor fiber. The left-most curve is the back-to-back link. The right-most curve is two-channel transmission through SMF-28 fiber. The slope difference is attributed to the dispersion of the fiber (20 ps/nm.km at 1590 nm). The middle curve is twochannel transmission through MetroCor fiber (-2.8 ps/nm.km at 1590 nm).
(A.U.)
b)
a) back-to-back link
50 km SMF-28 fiber
Analysis We performed simulations to model the transmission system and verify the validity of the experimental results. Using the laser rate equations, we modulated the laser with a random pattern. We simulated propagation through fiber with linear dispersion 20 ps/nm.km and -3 ps/nm.km (SMF-28 and MetroCor respectively). Amplifier noise was not included. Next, we created an eye diagram with the propagated waveforms. To calculate the bit error rate, we fit 0 and 1 levels from the eye diagrams to a gaussian. Next, using the fit values for the mean and variation, we used the following equation to compute BER:
æ I1 − I 0 ö BER = Q ç ÷ èσ 0 +σ 1ø Finally, to generate the BER plots, we varied the received power keeping thermal noise constant. We verified the validity of our model by simulating at 2.5 Gb/s. Our simulation data agreed qualitatively with the experimental results. For the simulation results shown, we were interested in predicting the performance of 10 Gb/s systems. At these higher bit-rates, the effect of dispersion is more pronounced. Because of higher dispersion, the slope in the SMF-28 simulation was lower than at 2.5 Gb/s (not shown here). Figure 4 shows the simulated bit error plot for a 10 Gb/s signal, with a linewidth enhancement factor (α) of 2. Note that for low error rates, the SMF-28 eye diagrams did not fit the gaussian hence we could not determine the BER. Also, the error in the points comes from low number of
c)
50 km MetroCor fiber Figure 4: Simulated BER curves and eye diagrams
Conclusion In these experiments, we have demonstrated the feasibility of error-free 2.5 Gb/s WDM transmission using low-cost Lband VCSELs using SOA amplification. SOAs are suitable for high performance transmission systems, exhibiting a power penalty of ~1.2 dB after 50km of MetroCor fiber, as compared to the EDFA. MetroCor fiber is well suited for directly modulated laser sources such as VCSELs. Acknowledgments The authors thank Dr. W. Yuen, and Dr. M. Jansen at Bandwidth 9 Inc. for providing the 1.55 um VCSELs used in this experiment. We thank Dr. B. Verbeek of JDSU Inc. for providing the SOAs. As well, we thank Dr. G. Doran and Dr. P. Dupriez at Corning Inc. for loaning the fiber. References /1/ /2/ /3/ /4/
Yuen, W., et. al., Elec. Lett., Vol. 36, 13, 1121, 2000 Stone, R.J., et. al., Elec. Lett., Vol. 36, 21, 1793, 2000 Wang, C., ECOC 2000, Vol. 1, pp. 97-98 Chang, C.-H., et al., ISLC 2000, pp. 95-96.
