Semiconductor Laser Signal Modulation Characteristics

With its high frequency and high-speed direct modulation, semiconductor laser has become the most important devices in optical communication system. DC bias above the threshold conditions, the modulation signal loaded, the output of laser light can be modulated directly. The main research work is to study the large-signal semiconductor laser modulation characteristics.Theory research in modulation characteristics of semiconductor laser develop fast in the eighties of the twentieth century. From the end of the eighties to the early nineties, small signal modulation theory reaches maturity. The main research purpose of small signal modulation characteristics is to enhance the rate of optical communication and transmission performance. Seed source technology in the MOPA system is a research need of large-signal modulation in high-power high-speed laser. Nonlinear effects become more significant under large-signal modulation, leading to modulation rate (bandwidth) decline and frequency chirp which makes a wider spectrum, the quality of optical signals deteriorate.Numerical solution and the matrix transmission method are used in the solution of large-signal rate equation. In this paper, analytical methods of the density matrix is used to derive general rate equation, and then to derive the chirp formula. Numerical method is used to resolve rate equation. Then the general low-power DH lasers with quantum-well lasers of large-signal transient are compared. The results show that quantum-well laser with a lower threshold carrier density, higher optical output power, shorter photoelectric delay time, higher relaxation oscillation frequency.Then the rate equation model of SQW-LD and MQW-LD including the carrier transfer effect is established. The transfer matrix method is used to analyze large-signal modulation response , bandwidth and chirp effect of low-power laser. The transmission bandwidth of SQW-LD with TQW-LD is compared. Under the condition of different SCH layer width of SQW-LD, transmission bandwidth is compared. The wider the SCH layer, the more obvious carrier transport effects, which make transmission bandwidth reduced, the effects of chirp more obvious. Finally, on the basis of analyzing the modulation characteristics of the low-power quantum well lasers under large signal modulation, the large-signal modulation characteristics of high-power quantum well laser is studied. Modulation bandwidth characteristics and the chirp features are studied in detail. Low-power LD and high-power LD have similar large-signal modulation characteristics. Under the large-signal modulation, the chirp of low-power laser is in A0 magnitude. The greater of Pulse modulation current the larger of chirp, and the chirp of high-power laser is in nm magnitude. A number of important conclusions are summed up at last, which can be used to explain the performance of the MOPA system.