| High-speed vertical cavity surface-emitting lasers(VCSELs)have the advantages of high modulation bandwidth,low production cost,high coupling efficiency with fiber,and high reliability,therefore,VCSEL is the most suitable light source in the optical communication system.In recent years,with the rapid development of the information society and the advent of the era of big data,the requirements for the data transmission capability of optical communication systems have been further improved.Only continuous increases in the transmission rate of high-speed VCSEL can meet the increasing demand for data transmission.The transmission rate of VCSEL is closely related to its modulation bandwidth,and increasing the bandwidth of the device to increase its transmission rate is a very direct and effective method.However,with the continuous optimization of VCSEL structure,it is now difficult to greatly increase the bandwidth of VCSEL,so the use of more advanced modulation formats to modulate VCSEL has become an important way to improve the transmission rate of VCSEL.PAM4 modulation has become an important modulation format for VCSEL in the transmission chain due to its advantages of simple operation,low system requirements,and theoretically modulated VCSEL transmission rate can reach twice the transmission rate of traditional NRZ modulation VCSEL.However,as the transmission rate increases,PAM4 modulation also affects the quality of the signal transmitted in the link.Therefore,it is of high research significance to study how to use PAM4 modulation to increase the transmission rate while reducing its influence on the quality of the transmitted signal.This paper mainly focuses on the structure of high-speed VCSEL and its data transmission capability under PAM4 modulation,and hopes to improve the transmission rate and signal quality of VCSEL under PAM4 modulation in experimental tests.The main contents of this paper are as follows.(1)The research background and research status of high-speed VCSEL are briefly introduced,and the current research progress of PAM4-modulated VCSEL is briefly summarized.(2)The structure of VCSEL is briefly introduced,including DBR source region.Then the transfer function of VCSEL is derived from the rate equation of VCSEL.This paper briefly discusses the limiting factors of high speed VCSEL modulation bandwidth through transfer function,and analyzes how to optimize the structure of VCSEL to improve its bandwidth.(3)The theoretical knowledge of several advanced modulation formats is introduced,and the PAM4 modulation format is finally selected as the research content of this paper by comprehensively considering the achievability of modulation format and the improvement effect on transmission rate.Next,several methods that can be used in the transmission chain to improve the transmission signal quality of PAM4-modulated VCSEL are listed.(4)Static parameter and dynamic parameter tests were carried out for high-speed VCSEL.The VCSEL for testing is a high-speed 1550 nm VCSEL prepared by wafer fusion technology,and the difference between each VCSEL is the difference in the size of the buried tunnel junction(BTJ)pore size,from small to large,4,5,6,7 and 8 μm,respectively.Through the tests of light intensity-current-voltage(L-I-V),spectrum,small signal and relative intensity noise(RIN),the correlation between VCSEL structural parameters and static parameters and dynamic parameters was obtained,and considering the test results,we finally selected VCSEL with 6 μm BTJ aperture as the transmission rate test device,which has a light output power of about 4.5 m W,can achieve single-mode operation,modulation bandwidth of about 10.5 GHz and RIN noise is relatively small.Suitable for data transmission testing.(5)The transmission rate test of 6 μm aperture VCSEL is carried out,which can achieve a transmission rate of 30 Gbit/s through NRZ modulation and 38 Gbit/s by PAM4 modulation.To improve the signal quality during PAM4 modulation and further increase the transmission rate,we added pre-emphasis and equalization techniques to the transmission chain,and achieved a transmission rate of 50 Gbit/s.Then,the transmission rate of VCSEL under different fiber lengths was tested,and the transmission rate of 42 Gbit/s was achieved on a 2000 m fiber using PAM4 modulation and pre-emphasis and equalization technology,and the transmission rate of VCSEL was improved. |
