Abstract
Directly modulated 850-nm multimode vertical-cavity surface-emitting lasers (MM-VCSELs) with different oxide apertures and transmission microstrip lengths are compared on the transmission performance of the non-return-to-zero on-off keying (NRZ-OOK) and four-level pulse amplitude modulation (PAM-4) data formats. In this work, intrinsic and extrinsic responses of the MM-VCSEL are also discussed concurrently. By tuning the length of the transmission microstrip in VCSEL, the low reflection coefficient and the enhanced 3-dB modulation bandwidth are achieved. The inductance of the transmission microstrip in the series connection with the capacitance in the active region is optimized to reduce the power loss induced by imaginary impedance. The different oxide aperture sizes for MM-VCSEL are also studied to control the capacitance and photon density. More importantly, the 3-dB modulation bandwidth, impedance matching, slope efficiency, relative intensity noise (RIN), and mode partition noise (MPN) for the MM-VCSEL with various designs are discussed to determine the best device with the high-speed transmission capability. The optimal MM-VCSEL with a diameter of 7 µm oxide aperture and a length of 25 µm transmission microstrip successfully demonstrates 50-Gbit/s OOK and 84-Gbit/s PAM4 after using the pre-emphasis technique for future data-center applications.
Highlights
The demands for streaming high-definition videos and interactive-reality programs have increased year by year
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In this work, shortening the transmission microstrip length of 55 mm reduces the inductance in the equivalent circuit of the MM-VCSEL to improve the 3-dB modulation bandwidth to 18.98 GHz under 15-Ith operation
Summary
The demands for streaming high-definition videos and interactive-reality programs have increased year by year. For improving the modulation bandwidth of the MM-VCSEL, the microstrip transmission line is employed as the access line connection between the pad and the p-type metal contact of VCSEL to reduce the limitation of the low-pass filter induced by the substrate parasitic capacitance. The connected line and contact pad can be designed with a buffered or tapered impedance if there is an impedance mismatch in between, which can effectively reduce the feedback of the data signal to increase the modulation efficiency and decrease the power consumption. The transmission performances of the NRZ-OOK and PAM-4 data formats are compared in these MM-VCSELs with four designs
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