The relentless growth of global internet traffic, once dominated by traditional backbone service providers, is now determined by a new wave of traffic growth fueled by the latest applications targeting mobile internet, cloud computing, and social networks. The size and number of warehouse-scale data centers (WSDs) has increased rapidly over the past five years to meet this significant growth in internet traffic. An important issue that these WSDs face is inter- and intra-data center connections where performance, transmission capacity, power consumption, and cost are critical issues that need to be negotiated carefully. A mega data center can consist of multiple buildings, with the reach requirements for an intra-data center ranging from 10m to 10km. Traditional copper-based interconnects quickly reach physical limitations as signal frequencies approach 10GHz.
The active optical cable (AOC), composed mainly of vertical-cavity surface-emitting lasers (VCSELs) and multi-mode fiber (MMF), is considered the most promising candidate for high-capacity applications in short-distance fiber communication due to its low power consumption and wide-bandwidth performance. AOC operating at 10Gbit/sec has a maximum transmission distance of around 500m due to the effective modal bandwidth of the standard OM4 MMF (4700MHzkm). Various proposals have been recommended to extend this reach. Using matured 1300nm distributed feedback single-mode fiber is one quick solution, but the drawbacks of high-power consumption for edge-emitting lasers and small alignment tolerances for single-mode fibers drive research efforts toward finding ways to extend the reach of low-cost, low-power VCSEL-based MMF AOC.
Figure 1. (a) Conceptual cross-sectional view and top view of our vertical-cavity surface-emitting laser (VCSEL) device. (b) Characteristics of the optical output power (L) and voltage (V) versus the bias current (I) of our device. N: n-Type semiconductor. n-DBR: n-Type distributed Bragg reflector. P: p-Type semiconductor. PMGI: Polymethylglutarimide. Zn: Zinc.
To overcome the limitations of bit-rate transmission distance using MMF, we have developed a high-speed, 850nm wavelength VCSEL using oxide-relief and zinc (Zn)-diffusion1 techniques. The conceptual cross-sectional and top views of the device are shown in Figure 1(a). Our VCSEL structure has two unique features: the first is that our current-confined oxide layer has been removed (oxide relief). The air has a dielectric constant half that of the oxide layers, which leads to a further reduction in parasitic capacitance and an improvement in the optical-to-electrical bandwidth of the oxide-relief VCSEL.2 The second unique feature of our device structure is in the adoption of Zn-diffusion techniques on top of distributed Bragg reflector (DBR) layers to control the number of optical modes.1 Compared with reported techniques for transverse-mode control of VCSELs, such as surface relief structures3 and miniaturizing the size of the oxide aperture to less than 3μm,4 our Zn-diffusion near-single-mode VCSEL structures have the advantage of further reducing the differential resistance and eliminating the requirement for precise control of etching depths in top DBR layers.3 Figure 1(b) shows the light output, current, and voltage characteristics of the oxide-relief/Zn-diffusion VCSEL. Due to the Zn-diffusion process, the differential resistance achieved by our device can be as low as ∼140Ω.
Figure 2. (a) Measured values of -log bit-error-rate (BER) at 37Gbit/sec versus RF peak-to-peak driving voltage (Vpp) of the VCSEL under different bias currents. Energy-to-data rate ratio (EDR), heat-to-bit rate ratio (HBR), and nonreturn to zero (NRZ) data pattern information is also shown. The insets show the corresponding eye pattern under lowest bias current and Vpp operation, and (b) shows the measured eye patterns and corresponding operating conditions at 85°Cand maximum operating speed.
Figure 2(a) shows the measured back-to-back bit-error-rate (BER) versus RF peak-to-peak driving voltage (Vpp) of our device under different bias currents at 37Gbit/sec operation. The inset shows the corresponding error-free oscilloscope eye patterns under the operating conditions with the lowest power consumption. As shown, we achieved an error-free 37Gbit/sec eye pattern under a 2.4mA bias current using an extremely small (0.5V) Vpp.
Figure 3. The measured–log BER at 25Gbit/sec through OM4 fiber transmission versus the received optical power (receiver end) and injected bias current (transmitter side) of the device.
High-speed performance of VCSELs under high operating temperatures is a further requirement for next-generation AOC. The values of heat-to-bit rate ratio (HBR) for our devices under different data rates of operation are shown in Figure 2(a). The achieved values for both energy-to-data rate ratio (EDR) and HBR are the lowest among all reported high-speed VCSELs at ∼40Gbit/sec operation.5,6 The device remains error-free at 33Gbit/sec under operating temperatures of 85°C,and a bias current of 2.8mA can be achieved: see Figure 2(b). Figure 3 shows the BER, measured at 25Gbit/sec operation, versus the optical power and injected bias current of our device, respectively. The MMF we adopted is the standard OM4 fiber, and the maximum error-free transmission distance can reach 0.8km. The corresponding energy-to-data rate distance ratio (EDDR) achieved by our device is as low as 175fJ/bitkm under 25Gbit/sec operation. To the best of our knowledge, this number is the record reported for all VCSELs7 for 25Gbit/sec data transmission through MMFs.
In summary, we have created a VCSEL device using oxide-relief and Zn-diffusion techniques. This device can be employed in OM4 MMF AOC for low-power, high-speed, low-cost data transmission. Due to the reduction in differential resistance and the increased control of optical modes in our unique VCSEL structure, such a device could further break through the EDDR limitations of high-speed VCSELs without the requirement of a miniaturized oxide aperture. Our next step is to further optimize the geometric structure of Zn-diffusion apertures to achieve pure single-mode operation, which could reduce the spectral width and increase the maximum possible transmission distance in OM4 fiber under the full range of bias current.
Jin-Wei Shi, Zhi-Rui Wei, Jhih-Min Wun
Department of Electrical Engineering
National Central University
Department of Photonics
National Chiao-Tung University
Department of Electrical Engineering
National Taiwan University
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