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Wireless microwave communication and optical fiber communication are the most important ways for communication.And the requirements are becoming higher by the development of modern society.Because of the free space optical communication have advantages that all the two methods get,it gains much more focus than ever.Free space optical communication is based on laser signal modulation,so it's meaningful to research on laser signal modulation.There are 2 types in laser signal modulation in free space optical communication system,the external modulation and the direct modulation.Commonly,external modulation is used in optical fiber communication systems,while direct modulation is used in free space optical communication system.This paper is focused on the research of laser diode direct modulation.Including:(1)Made analyzes and compares the symbol emissive power,bandwidth and bit error rate of on-off keying modulation,pulse position modulation and differential pulse position modulation.(2)Using the chip PT2262IR design circuit to achieve the on-off keying modulation laser signal,and through the chip PT2272 to demodulate the signal.(3)Design and implementation of the 16-PPM modulation and demodulation of the communication system,this system used the signal that the digital signal input by the ultrasonic distance measurement module,using FPGA as the main device of modulation and demodulation.Using Verilog to write code to control the logic array of the FPGA,and complete the modulation and demodulation of the signal.Complete the FPGA modulation module and demodulation module for time-series simulation.