Research On Broadband Tunable Frequency Locking Technology Of Semiconductor Lasers

Under consideration of system cost and complexity,how to produce high-performance microwave signal has become one of the key issues that need to be addressed in ROF system,fiber-optic sensing system,microwave photonics etc.However,generation of a microwave based on the electrical methods has shortcomings of complexity and high cost.Due to high frequency attenuation in electrical circuit,a major challenge exists in the development of microwave generation towards high frequency.Thanks to ultra-broadband nature in optical domain and low transmission loss in optical fibers,photonics-based generation of a microwave signal with wide bandwidth boosts great interests,it can be long-distance transmission due to a small loss in the optical fiber and has a very wide bandwidth.This thesis proposes and demonstrates a tunable frequency-locking scheme for two distributed feedback lasers(DFB-LDs)so as to generate a microwave wave at X band(8 GHz to 12 GHz).Two semiconductor lasers are synchronized and frequency-locked via optical phase locking loop(OPLL).This scheme can be used not only for ultra wideband transmission over a long distance in the fiber-optic time-frequency transmission system,but also for distributed fiber-optic sensing system and microwave photonics.The main content includes the following two aspects:(1)We optimize the measurement and feedback control of beating frequency between two DFB-LDs.The frequency and phase is measured by use of a low-noise chip(ADF4159)with the function of fractional dividion.By tuning the division ratio of ADF4159 via FPGA,the driving current of the slave semiconductor laser is precisely controlled through a feedback loop,so as to adjust its output frequency.The experimental result shows that the scheme enables the locking of beat frequency with good accuracy.(2)An electrical design of frequency locking and control functions is demonstrated.The control circuit of FPGA is also integrated,which improves the low-cost and stability of the electrical circuit.Frequency discrimination module,FPGA feedback control loop,temperature,and current driving circuit and two semiconductor lasers can be laid out on the same piece of a PCB board.The frequency locking experiment verifies the improved locking performance and the tunability of the beating frequency with a broadband frequency range.