The Vertically Integrated Transceiver For Single-fiber Bi-directional Optical Interconnects

The rapid development of the data centers and the high-performance computer makes high-speed short-distance optical interconnects technology,applied to the short-distance data communciations and the computer connection,obtain a great of attraction in recent years.Compared with the traditional cable connection technology,the optical interconnects technology has advantages of energy efficiency,fast switching,wavelength division multiplexing and parallel transmission,high reconfigurability and fewer hops.Therefore,optical interconnects technology has absolute advantages and will replace the electronic interconnects of all network architectures in data centers.Currently,the main solution to the short-distance optical interconnects is the Vertical-cavity Surface-emitting Lasers(VCSELs)and multimode optical fibers,due to its adavantages of low costs,low energy consumption,and high bandwidth density.Hence,VCSELs and multimode optical fibers will continue to dominate the short-distance optical interconnection market in the future.In order to build the next generation of high-speed data center network architecture and further improve the transmission rate of short-distance optical interconnects,based on short wavelength division multiplexing(SWDM)technology,multi-channel transmission has become a research hotspot because it can greatly decrease the costs of optical fiber.Moreover,400G BiDi MSA group and IEEE 802.3cm group have made the standards of the single-fiber bi-directional transmission technology.This technology is more popular because this solution has high forward compatibility and can support 400G network architecture based on the existing infrastructure directly.Curently,The BiDi transceiver on the market adopts a spectroscope to separate the detecting and transmitting optical signals with different wavelengths.This transceiver requires the precise optical alignment system,which leads to higher packaging cost.Besides,some researchers have integrated transversely the VCSEL and PIN photodetector(PINPD)to construct a transceiver.However,the coupling efficiency of the VCSEL unit and PINPD unit with the optical fiber are low,only 70%and 60%,respectively.Therefore,to solve these problems,a variety of vertically integtated transceivers chips are proposed,applied to the single-fiber bi-directional communicaitons.Compared to the separate devices,this transceiver can effectively improve the bandwidth utilization of optical fiber and reduce the number and cost of optical fiber.Compared to the BiDi transceiver,this transceiver can save the installation process of spectroscope,and effectively save the packaging costs.Compared to the transversely integrated transceiver,this transceiver can effectively improve the coupling efficiency of devices and optical fiber to 90%.Based on the advantages of VCSEL and multimode fiber scheme,this vertically integrated chip is expected to be an energy efficient,low package cost,high bandwidth density transceiver,applied to single-fiber bi-directional short-distance optical interconnects.The main research contents and innovative achievements of this thesis are as follows:1.The VCSEL-PINPD vertically integrated transceiver is proposed.The research contents of this transceiver from theoretical structure design,device performance simulation to frabrication and experimental verifi cation have been completed.It has fully proved that the transceiver is practical and has made foundation for the application to the short-distance optical interconnects technology.(1)A new structure of cavity-in Distributed Bragg Reflector(DBR)is proposed to solve the optical decoupling issue between the VCSEL unit and PINPD unit.By adding periodic DBR structure in a low-Q resonant cavity and respectively setting the center wavelengths of the cavity and the DBR at the transmitting wavelength and the detecting wavelength,the cavity-in DBR structure can provide the high reflectivity at transmitting wavelength and low reflectivity at detecting wavelength simultaneously.In the simulation design,the reflectivity at transmitting wavelength can be 100%,the transmissivity at detecting wavelength can also be 100%and the detecting wavelength range can be up to 20 nm when the transmissivity is higher than 85%.(2)The device structures of a matched VCSEL-PINPD transceiver have been designed.Besides,the static and dynamic characteristics of VCSEL unit and PINPD unit and the effects between these units at optics and electricity have been researched and analyzed by simulation.This pair of the VCSEL-PINPD transceivers has two working wavelengths of 848.1 nm and 805.3 nm.For VCSEL units,the threshold currents are 0.8mA and 1.1 mA,the slope efficiencies are 0.81 W/A and 0.86 W/A and the 3 dB modulation bandwidths are 15.1 GHz and 10.2 GHz.For PD units,the width of the quantum efficiency(QE)are 15 nm and 13 nm when the QE is higher than 70%and 3 dB response bandwidths are 23 GHz.The simulation results show that these two units can work independently at the same time and the transmission bandwidth can up to 10.2 GHz with good static performances.(3)The vertically integrated VCSEL-PINPD transceiver(emit at 850 nm and detect at 810 nm)has been epitaxial grown and fabricated.And the static characteristic of the VCSEL unit with cavity-in DBR structure has been researched and analyzed.The threshold current is 3 mA and the slope efficiency is 0.84 W/A.The VCSEL unit is the critical unit to decide the whether the transiceiver can be relazied,so the result above indicates that the VCSEL-PINPD transceiver is practical and also makes the foundation for the application to the short-distance single-fiber bi-directional optical interconnects.2.In order to further match the SWDM standard and make full use of the advantages of OM5 fiber,such as small transmission loss and long transmission distance,a new and vertically integrated VCSEL-RCEPD(Resonant cavity enhancement photodetector)transceiver is proposed.The absorption wavelength of the RCEPD can be extended to 900 nm or even longer because InGaAs/AlGaAs quantum wells can be used in the absorption region.The structure design,the simulation of the static and dynamic characteristics and the effects between the VCSEL unit and the PD unit at optics and electricity have been finished.For VCSEL unit,the threshold current is 1.68 mA,the solpe efficiency is 0.58 W/A and the 3 dB modulation bandwidth is 12.8 GHz.For the PD unit,the width of the QE is 8 nm when the QE is higher than 50%,the highest QE is 60%,and the 3 dB response bandwidth is 65 GHz.At optics,the effects between the VCSEL unit and the RCEPD unit are very small;at electricity,the limited bandwidth is 112 GHz when the isolation is set at-40 dB.Therefore,the two units can operate simutanously and separately.The VCSEL-RCEPD integrated device makes a certain foundation for the design of integrated device that makes the working wavelength red shift.3.To expand the width of the QE spectrum under the condition of higher QE,the VCSEL-coupled cavity RCEPD(VCSEL-ccRCEPD)vertically integrated device is proposed.The device structures of the matched VCSEL-ccRCEPD transceivers have been designed.Besides,the static and dynamic characteristics of the VCSEL units and the ccRCEPD units and the effects of the active regions of these units to each other-have been researched and analyzed by simulation.For VCSEL units,the threshold currents are 1.6 mA and 1.7 mA,the slope efficiencies are 0.74 W/A and 0.97 W/A and the 3 dB modulation bandwidths are 9.5 GHz and 11.0 GHz.For PD units,the 3 dB response bandwidths are 10 GHz,the widths of the QE spectrums are 8 nm and 6 nm when the QEs are higher than 60%and the highest QEs can be above 90%.Compared to the VCSEL-RCEPD,the QEs of the VCSEL-ccRCEPDs are increased by 20%-30%when their spectrum widths are close;and the widths of VCSEL-ccRCEPDs are expanded by 4?6 nm when their QEs are close.With the high QE,the broadening of QE spectrum can effectively increase the tolerance of laser temperature changing and optical link instability,and improve the practicability of vertically integrated devices.The VCSEL-ccRCEPD can make full use of the S WDM technology,and get better high frequency characteristics by optimizing the structure of the two units,so it can be better applied to the short-distance single-fiber bi-directional optical interconnects.