Research On High-speed Photodetectors And Their Arrays For Data Center Applications

The demand for network bandwidth of the data center has almost doubled within one and a half to two years.In order to cope with the continuous growth of east-west traffic,the data center has to upgrade the network system every three years on average,and the data transmission rate is also facing a rise from 4×100Gps to 4×200Gbps.At present,under the 400G Ethernet(400GE)standard,the system uses 4×100Gbps optical modules,with four-level pulse amplitude modulation(PAM4)used in each channel,and the bandwidth of the optical detector required to reach more than 36GHz.Under the next-generation 800G Ethernet(800GE)standard,the system uses a 4×200Gbps optical module,which requires the bandwidth of each channel of optical detectors to reach more than 56GHz.It is difficult for the next-generation 800 Gb/s data center to meet the photodetector performance requirements of optical interconnection just with the normal incidence PIN photodetector.The Uni-Traveling-Carrier photodetector(UTC-PD)whose transit time is effectively suppressed lowers the impact of the hole transit time on the bandwidth of the photodetector,and it uses high-speed electrons as active carriers,which greatly reduces the number of carriers in the photodetector.In a word,UTC-PD has excellent performance of high speed and high power.In this paper,we improve the structure of UTC-PD and conduct a theoretical analysis of the improved photodetector structure which can maintain high responsivity under low optical power injection.At the same time,it achieves a high response speed and effectively meets the requirements of optical interconnection for the performance of optical detectors of the next-generation 800 Gb/s data center.The innovations and main work of this paper are as follows:1.An Modified Uni-Traveling-Carrier photodetector(MUTC-PD)with synchronized transit times of photogenerated electrons and photogenerated holes is studied.The absorption region of the photodetector is divided into two parts:the uniformly doped region and the intrinsic region.By adjusting the doping concentration of the uniformly doped region,the transit time of the photogenerated electrons and that of the photogenerated holes are synchronized,which largely shorten the transit time of carriers in the absorption region and greatly improve the total bandwidth of the photodetector,with the bandwidth up to 52 GHz and the responsivity reaching 0.64 A/W.At the same time,the device can still maintain a high bandwidth under the optical power injection within 1 MW due to the introduction of the electric field into the absorption region of the photodetector.2.An Electric-enhanced Modified Uni-Traveling Carrier Photodetector is proposed based on MUTC-PD.The absorption region of the photodetector is divided into two parts:the gradient doped region and the intrinsic region.The electric field in the absorption region of the photodetector is enhanced by the gradient doping of the absorption region which is divided into a partial depletion region and a depletion region.Compared with MUTC-PD,the transit time of carriers in the absorption region is further shortened.The research results show that the MUTC-PD with electric fieldenhanced hybrid absorption layer structure can maintain a high bandwidth up to 60GHz with a responsivity of 0.64A/W under low optical power injection,which can be used in the next generation 800Gb/s optical communication systems.3.For a data center interconnect optical module with four channels,the receiver is based on a four-channel photodetector array chip.In this paper,the photodetector array chip is tested based on the frequency response test system of the lightwave device analyzer.It is shown that the bandwidth of the photodetector reaches more than 30GHz under different bias voltages.4.A microstructured optical lens is integrated on the photodetector substrate to improve the coupling efficiency between the photodetector and the incident light.The diameter and dome height of the required microlens are calculated by determining the distance between the AWG and the photosensitive surface of the photodetector.The thick glue process of the microlenses is carried out by the photolithography hot-melt method,followed by the dry etching method to etch the 18 μm dome-height lens.It is observed through the scanning electron microscope(SEM)that the prepared InP microlenses have complete morphology and smooth surface.