1.3 µm InGaAlAs/InP VCSEL for 10G Ethernet W. Hofmann1, M. Ortsiefer2, E. Rönneberg2, C. Neumeyr2, G. Böhm1, and M.-C. Amann1

1) Walter Schottky Institut, Technische Universität München, Am Coulombwall 3, 85748 Garching, Germany, hofmann@wsi.tum.de 2) VERTILAS GmbH, Lichtenbergstr. 8, D-85748 Garching, Germany, URL: www.vertilas.com

Abstract: 1.3 µm InGaAlAs/InP VCSELs for 10G Ethernet solutions are presented. High modulation bandwidth and error-free data transmission at 10.3 Gb/s up to 75°C over 10 km of SMF is demonstrated.

1. Introduction Vertical-cavity surface-emitting lasers (VCSELs) emitting in the 1.3 µm wavelength range have reached a degree of maturity allowing to enter industrial applications [1]. Smaller form-factors and lower power-dissipation standards in optical communication modules gear up the need for new generations of ultralow power long wavelength lasers. The 10GBASE-LR (long range) standard, to be found in the IEEE 802.3ae recommendation, describes 10G Ethernet at 1.3 µm over a link of 10 km of the widely deployed standard single mode fiber (SMF). For the 10GBASE-SR (short range) standard, 850 nm VCSELs are already used as cost-effective light sources. Recently, many efforts have been made to improve VCSEL performance at 1.3 µm wavelength including wafer-fused devices [2] and nitrogen con-taining quantum wells [3].

Our solution is a InP-based, monolithic approach, using a buried tunnel junction (BTJ) as current aperture as described in [4]. With this concept, we have demonstrated devices at 1.55 µm with superior high-speed performance [4]. Furthermore, we mounted our device into a transmitter optical sub-assembly (TOSA) module which can easily be integrated in existing transmitter infrastructure.

2. Structure and VCSEL Characteristics The basic structure of the present high-speed 1.3 µm VCSEL is basically the same as described previously [4] with optimized heat management in the cladding layers and improved bottom-mirror reflectivity. The epitaxial output mirror is made of InGaAlAs/InAlAs layer pairs and the active region consists of 7 strained quantum wells separated by tensile strained barriers. The hybrid back mirror of these lasers is made of 3.5 pairs of CaF2/ZnS and a gold layer. Laser chip and schematic structure is shown in Fig. 1. BCB is used as low-dielectric constant passivation to enable high-speed operation. For compatibility with commercial transceiver modules, the chip was mounted into a TOSA module which is also shown as inset in Fig. 1.

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Fig. 1. Schematic of the 1.3 µm VCSEL; laser chip and TOSA as inset. Fig. 2. L-I-V characteristics Fig. 3. Optical spectrum

The device presented here exhibits a single-mode output power exceeding 3 mW at room temperature and around 0.6 mW at 80°C as shown in Fig. 2. The fiber-coupling efficiency of the TOSA module is typically around 50%. The threshold current remains constant at 2.7 mA for 20°C and 80°C, as these devices for optical communications are optimized for high-temperature performance. The differential series resistance is around 30 Ω and well matched to the transmission lines of the driver modules. A single-mode spectrum at 1.33 µm is presented in Fig. 3. The side-mode suppression ratio was found to be at least 40 dB over the relevant current and temperature range.

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3. Results The small-signal modulation performance was verified on chip-level and is presented in Fig. 4. The small-signal frequency response measurements of the VCSEL-chip were done for various bias currents. The solid line is a fit to the three-pole filter function including relaxation-oscillation frequency, intrinsic damping and parasitics. Curve

fitting allows several intrinsic parameters such as modulation current efficiency factor and thermally limited maximum relaxation oscillation frequency to be extracted. The bandwidth exceeds 10 GHz at room temperature decreasing to 7.5 GHz at 80°C, both being sufficient for data transmission at 10 Gb/s. Following the 10GBASE-LR standard (IEEE 802.3ae), the transmission performance of the VCSEL-TOSA was evaluated under direct modulation at 10.3125 Gb/s with non-return-to-zero data (231-1 PRBS pattern length), over an un-amplified transmission link of standard single mode fiber (SMF) with a length of 10 km. With a matched photodiode receiver for the lab-environment, error-free received powers as low as -26 dBm were evaluated. To be closer to VCSEL-based communications solutions in the field, we inten-tionally used a bit error rate (BER)-detector module with lower performance for this experiment. Fig. 5

shows the BER measurements for 20°C (a) and 75°C (b). The corresponding open eye-diagrams are shown as inset. Error-free operation was achieved for all configurations and no error-floor was detected. Biasing conditions were set to achieve extinction ratios above 7 dB. Therefore, a slight over-shoot can be seen in the eye diagrams, yielding lower bit error rates.

Fig. 5. BER measurement BTB and over 10 km of SMF at 10.3 Gb/s (10GBASE-LR) at 20°C (a) and 75°C (b); Eye-diagrams as inset.

4. Conclusion We presented the transmission performance of TOSA packaged 1.3 µm VCSELs for 10G Ethernet. Error-free data transmission at 10.3 Gb/s over a link of 10 km SMF as described in the 10GBASE-LR standard is achieved up to 75°C. Together with their high bandwidth and low power consumption, these lasers are ideally suited for 10G Ethernet solutions at long range.

5. References [1] F. Koyama, Recent Advances of VCSEL Photonics, J. Lightwave Technol., 24, pp. 4502-4513, (2006) [2] V. Iakovlev et al., High-Performance Single-Mode VCSELs in the 1310-nm Waveband, IEEE Photon. Technol. Lett., 17, 947ff, (2005) [3] M. Yamada et al., Low-Threshold Operation of 1.34-µm GaInNAs VCSEL Grown by MOVPE, Photon. Technol. Lett., 17, 950ff, (2005) [4] W. Hofmann et al., High speed (> 11 GHz) modulation of BCB-passivated 1.55 µm InGaAlAs-InP VCSELs, Electron. Lett., 42, (2006)

Fig. 4. Small-signal modulation performance of 1.3 µm VCSEL

(a) (b)

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