Research On Monolithic Integrated Linear Modulation Dual-frequency DFB Laser Chip

Compared with traditional microwave radar,lidar has better range resolution and angle resolution.This leads to the widespred use prospects of lidar in driving,surveying and mapping,weather prediction and other fields.Because of the high integration and small size of semiconductor laser,lidar detection system with semiconductor laser as light source has become a research hotspot.The linear frequency modulation semiconductor laser has a high application prospect in the field of high precision speed measurement and ranging due to its large time bandwidth product and high integration degree.The common system structure to achieve linear frequency modulation signal have several drawbacks such as complex structure and high cost.It is particularly important to simplify the system structure of linear frequency modulation laser to reduce the cost.In this paper,a monolithic integrated two-section dual-frequency Distributed Feedback(DFB)laser and side-band injection locking technology are used to realize the linear frequency modulation dual-frequency laser output,which greatly simplifies the system structure,improves the system integration and reduces the system volume without reducing the detection range and range resolution.The main contents and results of this paper are as follows:(1)A linear frequency modulation generation system is manufactured.The system is based on the monolithic integrated two-section dual frequency DFB laser.By injecting a certain range of current into the master laser and the slaver laser,the four-wave mixing state is formed inside the laser.Phase consistency of the two modes in the laser can be improved by using the side-band injection locking technique to inject radio frequency signals whose frequency is equal to the frequency difference between main modes of the master and slave lasers.The linewidth of the laser is thus narrowed.Because of the phenomenon that the frequency change of radio signal tracted the frequency change of beat signal,the center frequency of the radio frequency signal equal to the half of the frequency difference between main modes of laser injected to the slave laser.The frequency of the master laser will vary linearly.Ultimately,the beat signal of the master laser and slaver laser will be a linear frequency modulation signal due to frequency traction.Experimental results show that this scheme can realize a lineary frequency modulation signal with center frequency of 19 GHz and modulation bandwidth of 1 GHz.Time-bandwidth product can reach to 5×10~4.When the power of the radio frequency signal was 11.4 d Bm,the beat signal can achieve a Pulse Compression Ratio (PCR)of 7.69×10~4.Due to the system preserves the high linearity of the radio frequency signal,the residual nonlinearity of the beat signal error is only 0.0468%.(2)A two-section dual-wavelengthlaser with grating reflector is manufactured.The grating reflector can provide additional reflection to the main modes of the laser leading to high sidemode suppression ratio(SMSR).Due to the different amplifting effect of grating reflector on the two main modes,the power difference between the two main modes can be balanced.The laser is designed and manufactured with Reconstruction-Equivalent-Chirp(REC) technology.The graphic size of the grating structure is micron.Therefore,the fabrication of the grating layer only needs ordinary micron lithography process to complete,which reduces the technical difficulty.The fabricated laser is experimentally demonstrated.The results showed that this structure can realize stable dual-frequency laser output and achieve sidemode suppression ratio higher than 30 d B at room temperature.Simultaneously,the power difference between two main modes remained below 0.5 d B.The parameters of the laser are still stable when the laser works for a long time.