CMOS optical 4-PAM VCSEL driver with modal-dispersion equalizer for 10Gb/s 500m MMF transmission

Data communication over 300m of distance, such as Ethernet standards, where the required data rate per channel reaches 10Gb/s or more, demand optical transmission [1]. Vertical-cavity surface-emitting laser (VCSEL) and multimode optical fiber (MMF) are usually used in such optical transmission systems, because they offer several advantages such as ease of assembly and optical alignment for cost reduction. The modulation frequency of commercial VCSEL devices is limited to approximately 10GHz. To overcome this limitation, solutions are reported in [2, 3]. However, these papers merely describe that the modulation frequency of VCSEL can be extended by equalization. On the other hand, the transmission performance of MMF is generally defined by the product of transmission distance and modulation frequency. In a commercial graded-index (GI) MMF, the transmission performance stays about 500MHz-km due to modal dispersion. In the case of 10Gb/s data transmission, the required bandwidth for transmission medium is 10GHz or more [4]. Therefore, when using these commercial GI MMF, the transmission distance will be limited to 50m. Thus, when several hundred meters of transmission is achieved using MMF [4], the modal dispersion of MMF becomes a rather predominant transmission bottleneck than the use of VCSEL . To overcome this bottleneck, a 4-PAM VCSEL driver with an equalization scheme to compensate the modal dispersion of MMF is described in this paper. The driver is fabricated in 90nm CMOS and achives 10Gb/s 500m 4-PAM transmission using conventional MMF having a 500MHz-km of transmission bandwidth. Using this driver, the transmission distance with 10Gb/s is extended by 10× (5GHz-km).